As greenoyster pointed out, you can insert pauses in the number to dial an extension.
Another thing you can do is to add a reminder to the contact with the extension in it. This won't dial the extension, but will bring it up any time you make a call. I do this for places where I need to use a different extension based on what department I need.
The reminder function is a slick one too, great idea especially if you have multiple possible extensions.
I love physical keyboards... but there is two devices that would make me consider a slab, one is something running a full version of Open webOS. The other is an iPhone!!!! HA HA just kidding (about the iPhone that is)...