ACT Airbus A321
Airbus does offer the A321 with either one or two Additional Centre Tanks (ACTs) in the cargo hold to add some range.
The structure does need to be reinforced to carry ACTs, however, and of course the fuel system needs to be adapted to accommodate them.
Only the A318 Elite (2 ACTs) , Airbus ACJ (up to 6 ACTs) and A320 Prestige (1 or 2 ACTs) are actively offered by Airbus as corporate jets today (they appear to satisfy most needs).
If a customer really wanted to have an A321 with 2 ACTs, then it should be possible.