Yes, there is a function called
removeQNs(MPS const& psi) that you can call on an MPS that will remove all of the quantum numbers and make the storage dense.
There isn't any function that will remove only some of the quantum numbers and not others, however.
I think for these more technical kinds of things, we would need help adding these features to ITensor which could be quite a project. It's something we might consider doing in the Julia version first if it would be used often, then porting back to the C++ version.
P.S. would you be able to do the application you want just with a regular dense MPS rather than having to keep the parity quantum numbers?