Well, in your case you can't promote a CPO to officer as all five officer slots are taken. You can promote one of your highest ranked sailors to PO though.
Hint: Just because a man isn't highlighted when you click on Promotions in SH3 doesn't mean that you can't promote him, provided of course that his experience is high enough.
Trust me, I've spent a *lot* of time playing with the crew.
BTW, don't you just love the "Not wanting to <whatever>, BUT..." qualification. LOL! This has been a topic of discussion in my office recently - there is someone that always says, "Not wanting to offend you, BUT..." and then promptly follows with an offence. We cut it off now at the "BUT..." by saying "well don't then".