From what I've read in the docs, I don't think this is possible in ACYMailing. I did find a not too convoluted way of doing it, which I'll describe here.
So we're starting with a MySQL query to select existing users to subscribe to a list. All we need is their Joomla user ID. The intermediary is Community Builder, which we user and which ACYMailing can use to subscribe users to a list based on a CB field value in the user's profile. So we create a field, numeric default NULL, and use the list of userIDs from the query above to generate the MySQL to set that field to 1 for those users.
Then ACYMailing will subscribe those users to the list based on the value 1 in that field. Not too difficult.
