not dangerous as much as it might not really do all that much good other then perhaps effect your accuracy. If the bullet is pure lead and pushed at a good enough speed the mushroom effect should come natural. If the bullet is not expanding as much as you like I would suggest instead to change bullet design....

There used to be a tip you could install on your ramrod and X a roundball. A different tip would star the ball. This was supposed to make it expand better and flatten out. My friends bought the tips and tried them but never seen a real difference between the expansion of their marked projectiles and the unmarked. They did claim a difference in long range accuracy. I myself never fell for the hype and just kept placing the roundball in the right spot and taking the animals that way....

You could try a few and see if it makes a difference. I do not think it would cause a danger to you. It might be an interesting experiment.....