With osu!mania skins, each mode requires its own section in skin.ini. Copy the settings from 7k mode, create a new section but change the key count to 8. Remember that each keymode has to start with [Mania].
[Mania]
Keys: 4
HitPosition: 482
KeysUnderNotes: 1
UpsideDown: 1
etc.
Remember to add 8th note images, column widths and whatnots.