Muti-Cell Fixtures default to "Minus Cells" in a partition both when creating a new partition with preexisting multi-cell fixtures and patching new multi-cell fixtures inside a preexisting partition. The cells cannot be added back into the partition. The current workaround is creating a completely new partition and adding only the multi-cell fixtures. It will add the masters & cells separately. Then you can add in additional Channels. You have to do this every time you patch new multi-cell channels inside the partition.
In a similar vein of "Minus Cells":
-Magic sheets seem not to have an option to control the entire multi cell fixture like you can from the command line. If the fixture is multi-cell, selecting it will post [Chan 1 Minus Cells] to the command line. There seems to be no way to create an object that just posts [Chan 1]. (I believe this issue was posted earlier)
-Querying a multi cell fixture type results in only the parent fixtures (ch 1 minus cells) being selected. Once that query is recorded into a group you can add cells separately, or subtract the channels and re-add