It depends on the context. You can build apps and reuse them or access their functions, UI, data etc. We already do this for things like generating IDs, re-using locations, do datetime calculations, have default asset data and so on. In this way, you (or anyone else) can build shared modules that can be used elsewhere.
The REI3 plattform itself does not have any plugins or extensions however. So if you want to have, for example, a JavaScript library embedded to use in your REI3 apps, it would need to be added to the source.