Yes, the adapters have the mounting holes for an N64 port. I already added one to the first adapter that I made (but with no N64 games to test with, I don't know whether the N64 slot works). Also of note: I never got around to epoxying my GBx adapter, and it works fine. Surprisingly sturdy.
I think I would want $12 per GBx adapter board plus whatever shipping method you want to pay for. Each individual extra board would be $3 (price I paid plus 60 cents). The blank boards come with the necessary resistor pre-installed.
I have 10 blank GBx/N64 boards available (nothing but the resistor soldered at the moment).
I'm located in the USA.
If you'd like to buy any, send me a direct message here (and I'll try to check back in a few days
after there was no response in the first few weeks I didn't expect anyone to be interested)
Note: If anyone else is interested, send me a personal message. If there's enough interest, I could try to scrounge up some more GBx plugs (hard to find these days) and make another order for boards.