Megan, I don’t think there’s a way to “pull in” more fields automatically.
The external/ghost task in Plan B (which reflects the corresponding task in Plan A) is still 100% part of Plan B. I expect its resources and costs need to stay zero to avoid double-counting those of the source task. Its custom fields all have to comply with the restraints of Plan B’s structure (e.g. lookups, roll-ups, formulas, etc.), which don’t necessarily match those of Plan A.
Conceivably you could write some vba code to copy the desired data from the source task (in Plan A) to its corresponding external task (in Plan B). That effort would not be trivial. I think the simplest way to view the data is to temporarily collect the interdependent projects into a single linked Master Project. In that structure the external/ghost tasks are all invisible anyway.