To add patches to a known location, use the dump
plugin:
parts:
patches:
source: some/directory/in/your/source
plugin: dump
organize:
patchfile.diff: some/known/directory/in/stage/patchfile.diff
prime: [-*]
application:
after: [patches]
override-pull: |
snapcraftctl pull
patch -Np1 < $SNAPCRAFT_STAGE/some/known/directory/in/stage/patchfile.diff
...