Caveat emptor: I'm not a boot fitter, but I have heard of the following as a solution.
If you need space around the ball of the foot, try cutting the outside material of the liner horizontally; just a single cut to create a slot. This will allow the inside of the liner to 'expand' through the cut slot. It might give enough relief.
Similarly, if you needed more length, cut a horizontal slot along the front of the toe.
If you need more volume in general, see if there is a removable layer between the boot board and the liner.
All my suggestions presume you are just looking for a quick cheap fix to get through the season. They would likely make the boot undesirable for others.
A shop could punch the liner and the boot for more space.