Whilst there are mechanisms to appoint a new Executive Committee (E/C) Member if a current Member vacates their position, I don’t believe that an new / additional Member may be appointed other than by the Owners Corporation at a General Meeting.
If you regard the matter as immediately important, your Secretary could convene an Extraordinary General Meeting to nominate and vote on appointing the additional E/C member, or if you merely want to give your new Owner an opportunity to involve themselves in proceedings, then you can invite them to attend E/C Meetings and to participate in discussions; they just can’t vote.