It's possible, but you'd need to modify the design of your database to implement the change. Are you up for that? (Do you know how to write scripts and create new layouts?)
I ended up writing a PHP script which takes a text file with fields and a CSV file with column headers and does the merge that way. Took half an hour and is a lot more flexible than Filemaker.
Unfortunately the boss has a thing for Filemaker and has told me to find a solution for this. Coming from a programming background I'm finding Filemaker's scripting to be limited and clunky at best, is there any way to get away from the "graphical script builder" for want of a better word and dive straight into the code that makes it work?
We've been asking for that (Text based script editor) for about 10 years or so...
From that background it shouldn't be that hard to set up. You would need to set up a layout that lists the contacts with an added selection field formatted with a single value check box. You pull up the list on this layout (can be a small floating window) and click the check boxes you want to add to the currently selected group.
Clicking a button can then perform a find for all such "marked records" and your script can then add them all to the group. I don't have that file open before me to see all the details, but it probably requires looping through the found records and creating a new matching record in a join table used to record group membership.