You can use P4101Z1 to upload a Branch to your existing Item Master records, by setting the Update Item/Branch field, ITBR = 2, and specifying your Branch is the Branch Plant field, MCU.
After you have uploaded your Items using P4101Z1, you run R4101Z1I, which will create an Item Branch Record for your Items.
Next, as has been suggested already, you can use the Item Branch Duplication Program, P41015 or R41826, to duplicate your Items' Branch Record to other Branches.
Hope this helps, please let me know if you need further info.