An ACJ, now I'm sure that's comfort!
As is Airbus 380.
C series business jet... yeah, no doubt inserting VIP interior and extra fuel tanks is trivially feasible. Just as it is on BAC 1-11, DC-9, MD-80, Boeing 717, Fokker 28, 70 or 100, or Superjet 100 or ARJ21, or Boeing 737-200 or 727-100.
But while this gives better range than the baseline regional jet, the range remains worse than that of dedicated longhaul business jet.
Compare Fokker 100 against Bombardier Global Express. MTOW is 43,4 t for F100, 44,5 t for Global Express. Both have wingspans slightly over 28 m. The range of Global Express, however, is much longer: F100EJ can achieve just 3200 nm, which is more than the airliner range of 1700 nm, but less than the 6300 nm of Global Express.
The reasons are that F100 must haul much bigger fuselage with its drag and weight. The same fuselage, however, offers extra comfort and accommodation space.
Boeing Business Jet and ACJ offer roughly the same range, in their VIP versions, as do GLEX and GV, and much more accommodation volume - but at higher drag and fuel cost.
If the GV/GLEX range is not enough then what you need will be a widebody like B767 or A330, or bigger. Or conceivably VIP 757 or Tu-204 or DC-8.