It can be done with a bit of specialized training. Squeezing a ball of wax would only build crushing strength. To open the fingers you use the extensor muscles. Check out "expand your hand bands" and similar devices for training those specific muscles- along with extensor exercises you can find easily.

It would be very difficult (and risky) to do something like that. But maybe squeezing a ball of wax for grip strength will help.
Anyway, that scene is great.
Listen man. Don't worry about my green funk.