Favioring one repo over another

Was rebuilding Mycroft on another sd card for my raspberry pi for an experiment, this time I installed raspberry pi os as the base.

My question is, is it possible to add another repository but have apt only look to it when packages are not found in the stock one?

I don’t want to download packages from the new repository that can be downloaded from the raspberry pi repository. even if the raspberry pi ones are older.
Don’t want to break my build.