Is it possible to make a JW adapter powerglide flexplate for a 360 internal balance work on an external balance engine?

Possible? Yes.
Easy? Not really. You would have to know the correct amount of weight and the correct distance from the centerline of the crank, then add (or subtract) it in the right place.
Safe? Probably not and the cert on the flywheel would definitely be void.

When I put the Glide in with the cast crank motor, we just did an internal balance on the engine. Makes it easier for dampers, etc.

Yes, mopar performance used to sell the weight. Two ways. One was a tear drop peice that went between flexplate and crank. The other was just a weight to weld onto converter. You would weld to flywheel.

Actually find out the amount of weight. And remove it from the opposite position. A converter company like ptc could do this.

Jw will balance the flex plate for a 360. I just have to send it to them. You can order it from them balanced for a 360, just have to ask for it