You can't really "import" them into the list of available action buttons,
but you can certainly use any sort of image as an action button. The only
difference is that an Action Button automatically opens the Action Settings
dialog box and has the "Highlight Click" checkbox checked. Any other image
all you need to do is to right-click the object, select "Action Settings",
select the desired action, then make sure the "Highlight Click" is checked.
Works the same!