When you receive the meeting invites, you can edit the dial string to do all the work for you. Let's say 800-123-4567 is the call in number, and *123456* is the conf. number.

Edit the dial string so it looks like this:

800-123-4567 x*123456*

The BB will dial the 800 number, then a pop up screen will appear asking to dial the conf. number.

Better yet, tell the person who's sending them to you to send them out this way for BlackBerry friendly-ness.