You don’t have to be an owner to be on the EC, you just have to be nominated by an owner. So there is no issue with a non-owner being on the EC, either on a temporary or more long-term basis, i.e. as a result of appointment at the AGM. However note that you can’t have an owner being on the EC and also someone nominated by them, i.e. you can’t double dip.
In the case of a casual vacancy, the person appointed to fill that vacancy has to be appointed by the OC.
Refer Schedule 3, subpara 4(c):
2 Constitution of executive committees for strata schemes with more than 2 lots
(4) A person is not eligible for election as a member of an executive committee unless the person is:
(a) an individual who is an owner,
(b) a company nominee of a corporation that is an owner, or
(c) an individual who is not an owner but who is nominated for election by an owner who is not a candidate for election.