Welcome to the snapcraft community! 
As far as I know, the stage-packages property only supports installing packages using the system package manager like apt.
You must manually extract the deb file using dpkg in an override script [1] or use a separate part with the dump plugin [2]. Take a look here [3] for ideas.
For more details, you can always take a look at the snapcraft.yaml reference [4].
[1] https://snapcraft.io/docs/scriptlets
[2] https://snapcraft.io/docs/dump-plugin