Reportedly in their most recent discussions they have come to the conclusion that they cannot, for the reasons that Nym has outlined. The five carriage units could be lengthened by the addition of two pantograph cars, but not one.
To elaborate more;
It started with all 22x units being able to be electrodieselified, then Bombardier looked into it more and this got dropped in the next government announcement down to being for 220 units only, this ties in quite nicely with the biggest transformer I've seen in a rail vehicle (floating round the 2200kVA mark).
To add some context, the draw requirement on some comparable units:
Class 444: 1.5MW for traction motors, so likely to be a 1800kVA transformer that can be slotted in, same for the 450s.
The total power generation capacity of the Class 220 units at the moment is 2240kW if I'm thinking right off the cuff, and I personally wouldn't like to see anyone cram a bigger transformer under a rail vehicle with traction motors on it too...
Even without additional traction motors, you would need a 2.3MVA transformer, pendos have roughly 2.6MVA per transformer coach, and they're mahoosive for the pendos and result in nothing else being able to be in them carriages, so if you work from a ceiling of 2.5MVA (Less space under the 22x series) then even without incorporating traction motors into the new carriages, there simply isn't enough power from one transformer for anything more than a four carriage formation, so a five car formation would need two transformers totalling 2800kW, and that would be with two unpowered coaches, that isn't really wanted for running on leccy, so you're running up to needing 3880kW, just for traction motors, add in other bits and you're needing a 4.5MVA transformer capacity, but then you come to the problem of running 7x23m units, you could never couple these in service and run all the way to Chester as they're simply too long, you'd need to have everything with it's first call as Crewe or make extensive use of SDO at intermediate stations, potentially running onto Mid Wales, but it is very restrictive having this length, adding more transformer or unpowered coaches won't work as when on diesel it won't be powerful enough. You can't install more generator coaches as the QSK19F is very out of production. All of this would also require a HV AC Busbar across the top as running with two pantos isn't advisable for them to be up, hence the HV Bar on Class 390s.
Enough here for ye to realise why we won't see EDMU versions of 221s in the near future...?
(I personally DO know how it could be done, but no ROSCO or TOC would want it so it's a no go)
