I know that he time travelled, but obviousally there are rules to time travel. Otherwise Crono's co would have just time travelled to some point before he died, and saved him. But no, they needed to use some elaberate thing called the Chrono Trigger to bring him back from the void.
And as Chrono'99, The TD would already be in the DBT. I'm not exactally sure, but if Belthasar went back in time, that would change the future again, possibly making another Schala fall into the DBT, and another Schala for Lavos to drain.