Is it ok to enhance pipe insulation with fiberglass batts?
There's no concern unless there's moisture present. Fiberglass presents more of a mold trap than closed-cell foam. 

Keep in mind that you'd have to wrap the pipes carefully enough that no airflow can move around the insulation. In other words, just laying batts over the pipe does little if the ends of the run are open to the air. 

You might partially split the batt (think hotdog bun), slide it in place, and press the batt back together to create something of a seal.