When you say separately derived system, you mean feeding it thru the transformer?
Okay. To me that's not backfeeding it, but rather resupplying it as a subpanel (or separately derived system if using a transformer).
Optimally, use a 208V delta to 240/120 delta transformer... but it may not be a commonly stocked item. Minimum rating will depend on calculated load and connected equipment. For full 400A rating your looking at 167kVA minimum.
NOTE: Resupplied as a subpanel, you will have to isolate equipment ground, grounded neutral conductor, and GES. Resupplied as a separately derived system, isolation of the preceding will not be necessary.[/QUOTE]