The easiest thing to do for such cases is to add a note, perhaps with a picture of the building if you're comfortable.

There must be folk who dedicate their time to searching the OSM database for unresolved notes, because whenever I leave a note on StreetComplete mentioning an inaccuracy (e.g: a path where there isn't actually one, a driveway marked incorrectly as a road, etc.) I get an email alert a few days later notifying me that the note has been resolved - often by someone who lives in an entirely different country (and who can use the information I provide in the note plus the satellite imagery to resolve the note).

> One thing I wish it had was the ability to add an entirely new building.

If you want to add an entirely new building on the go, there is Vespucci:

https://f-droid.org/en/packages/de.blau.android/

But, honestly, adding a building on a phone sized screen, while quite possible, is much more easily done with either of the two desktop editors:

JOSM, Java based: https://josm.openstreetmap.de/

iD, Web based, this is the editor available from the "Edit" button from the OSM website: https://www.openstreetmap.org