I think you can take the shifter out and (slowly) fill it from the top. The problem is knowing how much to put in. Overfilling is a problem, and the specs aren't a lot of help (depends on how much drains out, whether it's the stock transmissions, etc).

These can be *really* tight, but I managed to get mine turned out eventually. When you put a new one back in, don't tighten very much. They don't need a lot.

To get rounded bolts out, use locking pliers (called "Vice-grips" here) and lock it with all your might. This is before you try to start turning it! Use a pipe on the wrench handle if need be. If the pliers unlock, turn them around so you are continuing to lock it as it turns. Sometimes using a hammer to bang the wrench handle can help.

You might also try a plumbers pipe wrench. These bite in as you turn. To get enough leverage w/o putting the car on a hoist, use a pipe to extend the handle.