Go to Addresses → Lists and select the target group you want to add to another auto target group.
With the following icon you open a menu with the option Add to auto target group:

In the next window you choose the auto target group to which the data shall be added. By this action only new addresses are considered: Addresses that are already in the auto target group will not be imported a second time. You have the option to mark the new addresses in the auto target group with a note. This note will be saved and can be used for filtering, e.g. if you select a certain registration date.