I'm creating an Instrument repeating group and I want to add a custom field to it – this works fine so far.

Instrument instrument = new Instrument(new Symbol("VOD.L"));
instrument.set(new SecurityID("GB012345689"));
instrument.set(new SecurityIDSource(SecurityIDSource.ISIN_NUMBER));
instrument.set(new SecurityExchange("L"));
instrument.setString(9001, "BLAH");

However, if I add that instrument group to another fieldmap, like
QuoteRequest.NoRelatedSym nrs = new QuoteRequest.NoRelatedSym();
nrs.set(instrument);

... then the custom field won't be copied. I know that regenerating QuickFix using the extended data dictionary would solve the problem, but this should not be required.

The problem lies in MessageComponent.java:

public void copyFrom(FieldMap fields) {
try {
int[] componentFields = getFields();
for (int i = 0; i < componentFields.length; i++) {
if (fields.isSetField(componentFields[i]))

}

This shows that copyFrom (used by FieldMap.set()) only copies the fields which are present in the data dictionary.