“If I had an opening”
In hiring, an opening is a vacant position. If I had an opening is hypothetical: Frank immediately explains that he checked that morning and no vacancy existed.
B1-35 / CEFR B1 / EPISODE 35
Giving a concise, specific reference
SceneA hiring manager is calling Frank for a reference. The candidate is an engineer he trained himself. Frank has notes ready.

WORDS & CONTEXT
In hiring, an opening is a vacant position. If I had an opening is hypothetical: Frank immediately explains that he checked that morning and no vacancy existed.
Take on something means to accept work or responsibility. Here Frank accepted extra work whenever asked, even when his own schedule was full.