Liberator is indeed stand alone, and PS won't recognise FITS.
If you want to manipulate FITS, load them into DSS. Stack them, then save the result as a 16 bit or 32 bit TIFF. Works perfectly. Even a pretend stack of 1 file WILL work - DSS won't do much, buit run through it's processes

Just leave out darks, lights, bias etc.
There is a FITS plugin for Photoshop, but the price is NOT pretty!!!