Maybe with a check to verify that the contact is in the roster ? So, if it
isn't, add an entry in the menu "Conversation". Or maybe in the contact's
informations dialog, but this would be less discoverable in this way.

One pro for the dialog, is that we'll be able to add a string like "This contact
isn't in your roster. Would you like to add it ?" or something. This kind of
information would indicate to the user why sometimes the option to add the
contact appears. The con is that it would'nt be very discoverable... But this is
the best way of doing it though.

Yeah, both ways has it's drawbacks. Having it in the menu would have the
drawback of the menu changing depending on the status of the remote contact.
That might not be a problem though.

Having it in the contact information dialog it could be placed where the
"re-request authorization" field is kept.

The issue is a bit different, but still very related :
In the Contact Information window of not added contacts, there's a Subscribe
button. But clicking on the button doesn't do anything.

I haven't debugged it further, but could if asked to. However, it seems very
simple to reproduce.

If this doesn't work it's the same issue as with all other jid where you don't
have contacts in your contacts list. Wouldn't be of much use to debug this, it's
waiting for my rewrite to land.

What I have done here is show the "add contact" option in chat windows (not
group chats) when the contact is NOT on the roster or your own user.

It seems to work pretty well.

WRT to the subscription button in the contact information dialog, that merely
asks to see presence, that does _NOT_ add someone to your roster.  Perhaps we
should have some distinction in the contact information dialog, i.e. a button
that either says "Resubscribe" or "Add this Contact".

This bug is now fixed.