How do you reasign the hotbar?

I'm leveling a tank class now and want to be able to drag the number symbols to the hotbar and have those buttons be usable with the number pad on my keyboard.

How do I reassign my hotbar to match my number pad?

All of that can be set in your keyboard mapping settings from the main menu. You can assign very specific keys to very specific hotbar slots by hotbar number and slot. It's pretty easy to figure out.

First, setup a hotbar with the items you want... drag those items in there.
Note the number of the hotbar and then go into your key mappings and assign a key to each of the functions on the hot bar you just setup.

As far as I know, this should work fairly easily. I've got a key important actions mapped to certain keys as well as two of my mouse buttons.