If you add something like this:
ns_section "ns/server/${servername}/acs/database"
ns_param availablepool pool1
ns_param availablepool pool2
ns_param availablepool pool3
Your OpenACS 4 instance will only use those three pools. Any other pools you define can be accessed via "ns_db gethandle" and can access another RDBMS without interference from the toolkit.
I've not used this to move data from Oracle to PG, for instance, but have used it with Musea's script to move bboard items from 3.2 to 4.x. I open one pool for my existing 3.2 installation and three for OpenACS 4 and then run the scripts to migrate data.
Should work fine if the other db's Oracle, too ...
