Imagine this scenario:
1. You already own Black76 and nothing else from IK
2. You purchase MODO Drums in the group buy
3. You sell your Black76 license for $10 (plus whatever the cost of the transfer if any - not sure what IK's policy is)
4. In the Promotions tab, you select Black76 as one of your 15 products (Note: it's now NFR)
...that should be totally possible within the rules of the Group Buy.