In case this is not obvious to anyone reading, this is a terrible, terrible idea. Discounting the obvious risks of being killed, or injured, if you are captured as a member of a foreign paramilitary unit, you will not be afforded the same legal protections as a prisoner of war. Even after this war ends you may still find yourself imprisoned. You risk being tried as a war criminal, or even just a regular criminal being charged with murder, or another offense against the state.
This has been done before by Bosnia during their conflict with Serbia. They made a call to Mujahideen of the world to defend them. The prize for participation was a Bosnian passport. Hindsight may be 20/20, however I do not see any possible scenario where Ukraine joins the EU after the dust settles. Even if Ukraine grants you a passport, it won't be worth much. Especially if Russia considers you a war criminal for your efforts.