I am trying to refill the transmission fluid in my gearbox, it is the type of EL that doesn't have a dipstick, anyway the bolt that you are meant to undo to refill it just won't come undone, I think I must have done it up too tight last time. So my question is, is there any other way to fill up the transmission?

very common problem, more so if you over tighten the plug when the trans is hot, and its in a bugger of a place to get a good swing on it.
Keep trying to get the plug out, we've had to hammer and chisel a few to get them out.

But it is starting to burr. So I really dont want to round it, is there anywhere else I can fill it from?

no, nowhere else to fill it from

try a metrinch spanner or hammer and sharp chisel.
if you get stuck, run it down to ossie park and see the boys at top quality trans and see if they can crack it for you