"It is possible, but illegal, unless you own both games."
Nope, even if you have license to both games (not "own", you know almost every EULAs say "This software is licensed, not sold"
), you still can't do that legally. Just see the EULA.
the CS 1.6 mp.dll and CZero mp.dll are actually the same, Valve just use GET_GAME_DIR() and strcmp() to detect if the game directory is "cstrike" or "czero" to decide blocking the bot code or not. So if you want the CZero bot to be used in CS 1.6, you'll have to modify the dll file (which is forbidden).