For the STALKER models is not illegal. Please read here (is in Russian) and you will see that a moderator on the official GSC Game World forums said it is ok to use STALKER resources as modifications for other games, non commercially, if proper credit is given.
For the SOURCE models there are many ideas, some say it's illegal to others this is legal. But it seems that it is not illegal exported the models from the Source games, just give credits.