Well, there might be an easier way, but I made it happen like this:
Create a Bar Group just for poisons and whitelist every poison.
Blacklist the poisons in your regular buff Bar Group, so they don't show up twice.
Place the poison bar group where you want your buffs to be and attach your regular buff bar group to the last bar of the poison bar group. (In Layout - Attachment - Bar Group; and be sure to check Last Bar)
Hope it helps