Can dark matter be converted to dark energy? According to energy mass conversion law,  mass can be converted to energy. So is it possible to convert dark matter into dark energy?  
 A: This is a misconception. Theoretically, Dark Energy is a form of energy which accelerates the expansion of the universe, whereas Dark Matter is thought to be made up of weakly interacting particles (which account for most of the matter in the universe). Dark Matter and Dark Energy have completely different properties, and their names don't imply that they should 'necessarily' be  inter-convertible. 
A: Mass and energy are the same thing. If you have a sealed internally reflective box of photons, it will have inertia and a exert a gravitational pull on any other mass/energy that may be around.
http://usersguidetotheuniverse.com/?p=2865
That being said, no properties of dark matter or dark energy are known. These terms are used to refer to matter and energy that "must be there" due to the "total" matter/energy inferred through indirect observations not agreeing with the directly observed matter/energy in the universe.
To determine whether there are processes that can convert dark matter into energy, we'd first have to discover what dark matter is.
A: In cosmology, when you apply the Einstein equation to the whole [homogeneous and isotropic] universe, you get the Friedman-Lemaître-Robertson-Walker (FLRW) equations. These equations dictate how the size of the universe evolves depending on its material content. The different components are characterized by their equations of state, the ratio between pressure and energy density $$ w = \frac{p}{\rho}$$
Non-relativistic ("cold") matter has $w=0$, relativistic ("hot") matter and radiation $w = \frac{1}{3}$, and Einstein's cosmological constant $w = -1$.
Cosmological observations from different sources (mainly cosmic background radiation, but also baryonic acoustic oscillations, supernovae...) had discovered to "dark" (i.e., not directly observable) contributions to the total mass-energy of the universe. One has $w=0$, and it's called dark matter. The other has $w \approx -1$ (to experimental resolution), and it's called dark energy. Thus, dark matter behaves gravitationally as "normal matter", while dark energy causes accelerated expansion.
That is, more or less, all that we know about dark matter and dark energy. It is hypothesized that dark matter is formed by weakly-interacting particles (or non-interacting at all), whose interaction we are not able to detect, while dark energy is associated to a constant energy density, maybe the point-zero energy of fields. That is the $\Lambda CDM$ (cosmological constant-cold dark matter) model, the actual consensus theory of cosmology.
We don't know if dark matter interacts, or how those interactions are. So the answer to your question is:
Who knows?
A: Dark matter are not related in that way. Dark energy is the energy responsible for the accelerating expansion of the universe. Dark matter is a source of gravitation which we couldn't see directly.
A: What happens when Dark Matter interacts with Dark Matter?  Nobody has a clue.
Nonetheless, Dark Matter 'annihilation' is invoked in many popular theories.
A lot of people here seem to have convinced themselves that the OP's question is implausible.  I think its inspiring.
(what follows is highly speculative, and so its likely wrong) 
Dark Energy could be the result of an outward 'pressure' caused by interactions of Dark Matter with itself.
Such an interaction could not possibly be one of the four fundamental forces we know of (strong, weak, EM, or gravity).
Dark Energy could be an outward 'pressure' that only interacts with Dark Matter, and blobs of regular matter are pulled along with blobs of Dark Matter through the force that they do share (i.e. gravity).
A: I agree with atsby, and would like to say that Dark Matter and Dark Energy are two fundamental concepts of which we know very little about. However I doubt that Dark Matter can be converted to Dark Energy the same way we can convert normal matter to energy, let alone convert Dark Matter to Dark Energy at all.
For starters, Dark Matter is the substance physicists believe is one of the reasons for gravitational force. Dark Matter has a tendency to pull all mass in the universe together. Dark Energy on the other hand, we know far less about (we don't know much about either in the first place). Dark Energy is believed to be the opposing force to Dark Matter, and has the tendency to pull all mass in the universe apart.
In my opinion, I strongly doubt that these two concepts, which have wildly differing properties, can be related in such a way which allows one to be converted to the other. However, at this point in time, we know very little about Dark Matter or Dark Energy, so who knows? Maybe in the future, we will find a way to convert Dark Matter to Dark Energy.
A: Let's rephrase the question: Is it a plausible hypothesis that the hypothetical energy responsible for the accelerated expansion of the universe might exist relative (like two sides of the same coin) to hypothetical loosely interacting particles that hypothetically comprise a large proportion of the mass of the universe? (ie., DE = DM C2).
Stated that way I can't see what it is not at least plausible. Find a way to do the math and you’ll probably get a phone call and large cashier check from someone in Stockholm.
