not sure it works like this (last time i checked, adduser didnt support additional groups with the extrausers setting) … but yes if it would work, you could indeed use it on development images …
but that wont help with production, there is no way beyond manually via ssh login to call adduser, the only properly usable interface for adding a user to core is via snapd and its snap create-user command or via system-user assertions …
also note that core is by design not really created for multi-user setups.