What do you mean by "add more space on the hotbar"?
If you hit ESC in-game, and go to Options, you should see a selection called "UI" somethingorother. Go to that option and you'll be presented with a screen which allows you to customize your 5 hotbars.
Also, while in-game, you can hit ctrl+\ to bring up your UI placement settings.
I hope this helps, I'll be back later (after I can get in-game) to better identify what I'm referring to.
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