I asked this question a couple years ago and, at the time, it wasn't possible, but I wondered where we are at adding a paying member in the admin panel.
At the moment, we find that the best way is to enter a membership as a new user in the front end (logged out) and then go to basket and pay it.but this is not 'efficient'.
As we now have to add data that cannot be seen on the front end, when registering, it would be easier if we could create/register a new user and then move on to the transaction in the backend!