Where are the hard points for attaching bombs to the wings? The 767 wing unlike the A330/A340 wing was not designed for 4 engines. The A330 MRTT uses the place where the A340 engine #1 and #4 pylons would be attached as the point where refueling drogue pods are attached.
The 767 cargo bay would have to be redesigned with a pressure bulkhead like the P-8 (737-ERX) for an internal bomb bay. I bet that would be both expensive and difficult to retrofit into an already existing 767. It makes more sense to use their F-15's and F-16's and use the 767 tanker to extend their range.
I doubt an F-15 could carry a 30,000+ bunker busting bomb - the limit is about 23,000 of external fuel/ordinance. I’m thinking a converted 767 could carry several. It would be a difficult re engineering project of the plane but not impossible.