Is it possible see an invisible light with something similar to a normal lens of an eyeglass? Let's say that I am a spy in a room with a lot of people and I want to communicate with my friend using Morse code via a light signal.
Does there exist a type of light (laser or otherwise) that is invisible to the human eye, but that I can see using a modified transparent lens? 
In other words, can I make an infrared or ultraviolet light visible using something similar to a normal lens of an eyeglass?
If yes, is it easy to buy or make it? 
 A: It is possible to partially convert infrared light or ultraviolet light into the visible regime by exploiting nonlinear optical phenomena.   Second harmonic generation (SHG) is one such phenomenon that doubles the lasing frequency using a crystal (e.g. it can convert 1064nm IR light to 532 nm green light).  
Most green laser pointers actually have an infrared diode laser that is converted to green light using simple harmonic generation.
This is easy to do, in the sense that a relatively cheap laser pointer can accomplish this.  However, in the case of the laser pointer, the laser diode is in a fixed position relative to the nonlinear crystal such that careful alignment can be maintained easily.
In your example, however, you require a light source on one side of the room and a crystal (attached to someone's eyeglasses) at the other end of the room.  Maintaining the necessary alignment would be extremely difficult.
A: Even better, you can buy pretty cheap equipment working on very low frequency lightwave that will allow you to see that light with your ears, so that you do not have to look and will not be noticed. It is called a radio. You can find small portable models.  Ask for a Talkie Walkie.
