Grun, if you're not a pilot, there's no restriction. If you're not on the seniority list in ALPA, you're not in the Union. So you can hold any of a large number of high paying positions working for ALPA.
Like I said, if you're real good, the Airline Transport Association will hire you away so they don't have to deal with you.
Even pilots that go to "management" like VP Flight Operations generally just go "inactive" but do not give up their seniority number. There's no requirement to resign from the Union.