I'm afraid it's not possible at the moment. Will see about adding it in a future version, just need to think of a reasonable way to add it to the UI.
EDIT: I've added this to the latest version. When hovering over the bar where you add the inputs, you can now press the + key to add more inputs to the group. These groups will have a display showing their decimal value. Still need to add an option to show the two's complement value though.