Connecting to a custom database using db_connect

[Julchen]

Is there a sane way to access a custom database using vB's built in database classes?
The database is on the same local server as the vB one.
Is there a recommended strategy on using db_connect and select_db? Like opening a second connection to the same server or just switching databases on the same connection?
Is it even a sane idea to crowbar in a 2nd database that way?

Over the years I created quite a few extra pages and stuff but so far I've been using completly "seperate" code to make them run.
Since I'm in the process of migrating my 3.8 install to 4.2, though, I need to rewrite everything anyway and I'd love to, finally, create proper installable addons/plugins for everything.

Yet, all the data is in the custom database, and I don't feel comfortable merging it with the forum tables.