They can easily drop an FCS or MWC team to do it.
But who does ND drop from their 12 game schedules to make it happen? They already dropped what some thought were significant rivals (like vs Michigan) to make their new 5 game requirement against the ACC happen. Take along with that Michigan St, USC, Stanford, Navy that they don't plan to drop any time soon. They have a full 9 game slate right here plus many many more waiting to get games with them.
Heck ND can't even fulfill the remaining 2:2 with BYU that they signed because of the new ACC slate. I'd be happy if we eventually could get those and yes it would be cool to have those played in week 14/CCG weekend...