I don't have the link on hand (will search my book-marks) but I believe the best is to slowly increase the
salinity week by week. I'll look up by how much for you. If they're in a fresh-water community you could grab an extra cycled fitler and do this is a rubber-made tub or spare tank.
Sorry I can't find it but I'm guessing that the same rule for moving fish to brakish would be similar to converting the bacteria over as well. (When setting up brakish tanks some just slowly increase the salinity of the their freshwater set ups) I can't find the link about it anymore though...

I'll keep looking.
Found
this link read the part about acclimating brackish fish it might help you out.