Hi Uma,
pftools (which is part of the parflow repo) provides a script (pfmask-to-pfsol.cpp) that you can use to convert your mask to a solid file (no DEM file needed).
In case you want to run this from python, the subsettools package (
https://hydroframesubsettools.readthedocs.io/en/latest/) provides a convenient way to run this script from python. For this, you would need to read your mask.pfb to a numpy array (with read_pfb, which is a function in pftools as well).
Please let us know if you need more guidance for how to do this.
Best,
George