In the past, IJM has subtly discouraged people from using P2 in a 3880 because it was argued that it's not needed. You really need P2 in printers where it's hard to change between PK and MK. In some of the older large format printers (7600?), there was only one black installed at a time, and so a switch between blacks wasted a lot of ink as the ink line and damper had to be flushed with the new black. It's certainly the case that the 3880 is not in that position - you can switch between MK and PK much more easily. So in that sense it's not needed. Which is why there aren't instructions on how to convert a 3880 to P2. However my previous comments still apply, i.e. while strictly speaking P2 is not needed, it's a much easier and simpler and more efficient way to run the printer. There's a thread here from another user who ran K7 in a 3880 and found the maintenance burden a bit large for his style of printing. P2 would have reduced those problems to more manageable levels.
To do the switch you'd need to put a cart with glossy shade 1 in the Yellow slot in place of the existing cart with shade 7. I haven't done this switch myself, but I think you could use your current PK cart and swap the chips over. You'd then need to put something in the PK slot. You could probably put the shade 7 Y cart in there with the chips from the PK cart, but a fresh cart with flush would be better. Once the inks are in the right slot, you then need to get the glossy shade 1 ink to the print head. Either you do two or three power cleans, which is guaranteed to waste a lot of ink from all channels, and fill the maintenance tank, or you print a lot of purge patterns using just that channel, which is possible using QTR Calibration Mode. Getting the ink to the head is the hard part.