Not clear at what point you intend to add it to the array. You should do this before you format it to XFS since unRAID will want a clear drive if adding it as an additional array drive. A clear drive is not at all the same as an empty filesystem drive.
So I should
1. empty cache drive
2. format it to XFS
3. add to array
4. start moving files from existing array drives to the XFS drive formerly known as cache
Assuming you don't want to do a new config and rebuild parity, you should preclear the drive, then add it to the array, then format it.
If you try to add a drive that is not clear to the array, unRAID will clear it, taking the array offline until it's done, then you will have to format it.
The only way to maintain valid parity when adding a drive is to add a clear drive. A clear drive is all zeroes, so has no effect on parity. A formatted drive is not all zeroes, because it has a filesystem written to it.
So, you should:
1. preclear drive
2. add to array
3. format