Well no.
The steward is a "replacement for the king, Since the middle years of the 3. age the Stewardship became hereditary. Therfor Boromir could not become a steward before Denethor died, sice Boromir died before Denethor he could not become a steward.
He was how ever captain of Gondor
(I have a wierd feeling that steward of Gondor has a nother meaning too! Otherwise it seems to obvios)
