upgrading regulator

Assuming the diagram is correct. ( i just don't know the 2 wire alternators) The blue and the blue with white stripe are at the same potential (battery voltage) The "F" green wire from the voltage regulator to the field of the alternator appears to be what regulates the field to regulate the charging voltage from the alternator.

So yes, you can join the blue and blue with white stripe wires together where ever you want. (at the ballast or anywhere else)