Hi Mushrooms! Sorry to hear you’re having trouble, but it seems unrelated to the importer itself. Imported palettes are saved in .GPL format by default which is typical for Aseprite, so they should behave just like any other palette. You may need to click the lock button to the upper-left of the palette list (or press “A”, which I think is the default shortcut) to enable “Edit Color” mode if you want to modify colors that already exist in a palette.
I just tried modifying a couple of imported palettes and things worked as expected. Let me know if you still have issues!