umm, well if you mean front seal in the front axle, then yeah, you could top it off and a little may leak out. I wouldn't go in any water or mud though, (i learned that one the hard way) if you mean front pump seal on your transmission, it will likely dump a bunch of tranny fluid and possibly not shift (come to think of it, I learned that one the hard way too, it cost me a torq converter) maybe worse. Don't get me wrong, I've been the king of unprepared wheeling, and even when you think you are ready, you can lose a seal like the one I lost on my tranny and have no choice in the matter in the middle of BFE.
As for changing front diff oil, you need to remove the fill cap and pump it out with some electric or drill pump.
Good luck.