Can electromagnetic fields be used as shielding for electromagnetic radiation?
The short answer is no. Shielding means total absorption and light, light is an electromagnetic wave, passes mostly through light without interacting at all. All frequencies of electromagnetic radiation have similar non interacting behavior.
Matter shields are a different proposition because depending on the energy of the electromagnetic wave different materials are needed. Metal shields from a lot of the radiation , a Faraday cage for low energy radiation. At light frequencies most materials block it. At x-ray and gamma ray a lot of material is necessary for the radiation to interact and become harmless.
Shielding electromagnetic radiation by electromagnetic field means achieving nonlinear interaction of electromagnetic field. This can be achieved in matter (say, using gas discharge in electromagnetic fields), but requires very high fields in vacuum (http://en.wikipedia.org/wiki/Schwinger_limit ). Fields that can be created using the most powerful lasers are currently a few orders of magnitude too weak.