Membership in a third order like the Third Order Franciscans would not be compatible with a vocation to Opus Dei. This is because membership in a third order is also considered a vocation.
However, members of third orders are always welcome to participate in the spiritual activities sponsored by Opus Dei and to become cooperators.
Members of third orders can join Opus Dei after leaving the third order under certain circumstances and with the approval of the prelate. The prelate may release them from any vows they took as part of membership in the third order.