I dont think serato sends midi clock so no i dont believe this is possible. I cant get djm900 to even receive midi clock from traktor so unless theres a really creative workaround, i dont see this being possible.
The only way i can think of is to bridge serato to ableton and then go from ableton to djm. Or perhaps use midiox as an intermediary for routing the bpm. But ull still have to get around any issues the djm has with receiving midi clock.
Is ur auto bpm not working so well? Mine is pretty hit or miss depending on the track.