adding an alternative power net in KiCad
Simply use global net labels, 5V_A and 5V_B are fine for the names. Go to "Place" -> "Global Label" and name it appropriately. If you use hierarchy sheets the labels will persist through all of them (Unlike a hierarchy label, which only has scope inside that sheet and you need to break out the labels on the layer above as sheet pins by right clicking on the sheet and going to "import sheet pins").

Another thing you can do is make your own library, and after modifying the 5V power port (as your example) you can "save to a new library" or other method of putting it into a new library.