Hi Andre!
The in-game mail system implementation provided by the SDK is indeed heavily opinionated towards the contents being generated by an admin via dashboard (either directly or indirectly via broadcasts). The actual client-visible part that it provides is simply a list of mail items and a couple of actions for mail-like semantics. You'll probably have a better experience isolating your friend requests into a separate list/queue in the player model and creating a specialized set of actions for mutating that state.
That said, there should be nothing stopping you from implementing a PlayerActor EntityAsk handler that accepts invitation requests from other players and sticks an an invitation mail-item into the mail inbox. You would then just maintain polymorphic mail content types internally by having separate classes that derive from
MetaInGameMail
and disallow creation of the "friend invitation" type of mails from dashboard.
HTH,
Antti