If it's the main panel, with the main disco, then that's where the neutral is bonded to the ground conductors, correct?
If you are adding another termination bar (neutral/ground) the bar you are adding should (IMHO) have a jumper connected to the "factory" bar. The "jumper" should be sized according to the incoming neutral that is connected to the "factory" bar.
If it's a sub-panel, then there has to be a "ground" bar, and an "isolated" neutral bar, with a "ground" and "neutral" conductor for each.
If the "added" bar in your scenario is "mounted" to the panel enclosure, then you are using the panel enclosure for a "neutral & ground connection back to the "factory" bar & bond. The current passing through other "grounds" depending on the resistances. Not Good.
Hopefully my explanation above is clear enough to understand.