I can't get the filler plug off so can I put the gear oil through the shifter hole?

Is the bolt stripped?

You can but without the plug, you won't know how much to put in and too much is just as bad as too little.

Hi:
They make special tools to remove stripped bolts. They are sockets. See if the place you have your oil changed has them. Or, you could check at a parts store for one you need. It's hard to describe, but the socket is designed to spin on to the nut and tighten as you loosten the stripped bolt.