It's not that Fritzing can't do it, it's just you haven't learnt how to do it.
Search reg
Right-click/Edit/Save as new part.
Right-click saved part/Edit/Meta and change it to 12V.
FZ was only designed to be a breadboard beginners aid - it currently errors when adding a SMD single-sided part -, it's everyone else that is pushing it further.
Inkscape is nothing like GIMP, in fact there is nothing you can learn from GIMP.
All I can suggest is my tutorials, as there is probably enough to make parts without learning INK.