Fritzing parts can be copied and modified. The process to get it right is finicky and not trivial. But definitely possible.
Depending what you really need, it is also possible to add pads and holes directly on the pcb view, without changing the part. In the parts window, select the “CORE” bin. Scroll down to the bottom, and have a look at the parts for “PCB view”. From your description, the “Hole” and “Pad” parts could do what you want.
If by “make a shield”, you mean design and create a pcb to be used as a shield, you may not need a modified part at all. In “CORE” parts, “Connection” section is a generic female header. The image looks like a single square, placing it gets 2 pins, but number of pins, form, pad shape, spacing, and more can all be changed using the Inspector window. A couple of rows of headers could provided the basic layout needed for the pcb. Though this way is more work to track which pin is supposed to have which function and connections. Connecting the header pins to the M0 in schematic view could help with that. Just ignore the ratsnest lines between the header and M0 part in pcb view when doing the trace layout. Keep the M0 part off of the pcb, so that Fritzing will not actually attempt to connect traces from the part to the headers (if doing any Autorouting).
EDIT: for layout purposes, an UNO R3 board part might also get what you want.