How to sweeten ganache for a chocolate lining
While adding sugar syrups like corn syrup or invert syrup to ganache is fairly common, it will change the texture of the ganache. You'd end up with something more like a chocolate coating. 

If bitterness specifically is the problem, I'd suggest using a less bitter chocolate. Perhaps you can find something with lower cocoa solids that will have a smoother flavor without adding any sweeteners.

Added from a comment since I remembered: A tiny amount of salt can also help to cover bitterness. You could try that.