Is a plug adapter considered an "extension cord" under the NEC?
I'm not sure the term "extension cord" appears in the NEC at all. There are provisions for "flexible wiring" (section 400) but those are probably not relevant for what you're talking about. (In fact, what are you talking about?)

The closest thing I can think of would be an "attachment plugs" and "cord connectors", which is discussed in section 406.7. Among other requirements are that they be listed. I would think that if you are field-assembling something out of existing plugs that as long as the plugs themselves are listed and the wiring conforms to the rest of NEC (suitable gauge, protection from damage, etc) that would be OK.