Would it though? At the moment for voyagers, you'd have the problem with engines, but it would be quite possible to get around if you wanted to do it badly enough.
There will be better ways of doing it than this but never mind!
If you classed the 220s and 221s as the same units for now to make life easier:
Virgin have 18 5 car voyagers and 3 4 car voyagers. That is 102 coaches. That means 102 engines
XC have 35 4 car voyagers and 22 5 car voyagers. That is 250 coaches. That is 250 engines.
If you made 44 coaches to lengthen all XC 5 car sets to 7 car and 105 coaches to make the 4 car ones 7 car you'd have 149 new XC coaches without engines. 102 of those could then be fitted with the engines from Virgin, leaving 47 still engine less. 42 of these could be fitted with a new type of engine, leaving XC with 6 non-standard (or are they?) full 7 car sets.
Now this is where it gets a bit (more) complicated:
You now still have 5 coaches without engines. 2 for 5 car set. These coaches would all be fitted with the new type of engine. 3 for a 4 car set. These have no engine for the time being.
The 5 car set, would have two coaches added, to make it 7 car, and then the 5 original coaches have their original engines replaced with the new type.
The 4 car set would have 3 coaches added without engines, to make it 7 car. The three coaches would then have the original engines from the 5 car set above.
This would leave two original engines for spares
The virgin ones would all just have new type engines fitted
Totals: I think I've gone wrong here
CrossCountry 7 car voyagers with original engines (220 & 221s): 50
CrossCountry 7 car voyagers with new type engines: 7
