if I understood correctly, you should check the "allow entry of other values" in the inspector for that field.
Put a trigger that fires when the field has been modified and go check if the value is an existing one in the base list, if not, create a record with it in the table the pop-up menu is based upon.
I'm not sure I understand. I currently have three tables. One is the baseball cards, one is the Sports, and the other is Manufacture. Sports and Manufacture are related to baseball cards and baseball cards gets its information from those two tables. What I'm trying to avoid is when I come to a card that its manufacture and brand are not entered in the Manufacture table, that I have to leave the Baseball Card table to go manually enter the information into the Manufacture table.