Sure.
Display the skills/potions pane by clicking on the appropiate browse button on the in game HUD, place the mouse cursor over the icon for the skill/potion you wish to assign the hoykey to, and press F1-F12, or CTRL+(F1-F9). The icon will be updated to display the hotkey assigned.
Note that CTRL+F12 brings up the assign keys screen, and CTRL+F11 dumps a screenshot, so these can't be used.