Yes, it is acceptable to overboard the existing floor with a layer of flooring grade chipboard so that the minimum depth of 38mm is achieved.  The joints of the new layer of flooring grade chipboard should be staggered with joints of the board already installed, and fixed through the existing boards to the galvanised steel joists using timber-deck winged self-drilling screws at nominal 400mm centres. 

Check that the galvanised floor joists will carry the additional weight of the additional layer of board and any associated increase in load which may act on the floor.