Stage-snaps with snaps on local harddrive

I can stage a snap from the snap store like this:

parts:
audacity:
plugin: nil
stage-snaps:
- to amd64:
- audacity

How can i stage a snap that is not in the snap store, but “locally” on my harddrive?

Background:
To increase the build performance of my snap, i want to shift some long running task (which are stable and do not always have to be built again) to another snap.

This is currently not possible and probably won’t be in the foreseeable future.