I just can't see how a dual power locomotive couldn't be easily adapted since the traction motor is electric.  You'd just need to rig on some feelers for the electric lines, have something that adapted the current to the motor current (maybe even in the electric lines themselves) and a controller to control the power output.  Presto, electric train.
IMHO, an existing diesel electric design could be adapted (and this is what should be done) to dual mode, but an existing diesel-electric loco would not be cheap to adapt.

Rearranging wires, adding a 3rd rail shoe or pantograph, mounting a transformer (where ?), new controls, etc. on an older loco would make me grind my teeth.

Far easier to do in a Cad/Cam program than to preexisting steel & wiring !

Best Hopes,

Alan

Might be a PITA, but unlike PO, that really would just be an engineering problem.  Have you ever contacted any of the locomotive manufacturers to see if they have plans on hand for conversion or whether they've done any research into it?  I would hazard a guess that someone's put some thought into it.
I have had several conversations with Larry Conrad, VP Engineering at Brookville Equipment, the last US maker of small locos.

Best Hopes,

Alan