Well I thought we couldn't do this (I remember seeing a feature request regarding this a few days ago)...
But if I understand correctly it goes as following:
From the skin.ini wiki:
//The sprite to use (mania-note1.png) for notes appearing in the first column
NoteImage0: mania-note1
//The sprite to use (mania-note1H.png) for the head of slider notes appearing in the first column
NoteImage0H: mania-note1H
//The sprite to use (mania-note1L.png) for the body of slider notes appearing in the first column
NoteImage0L: mania-note1L
//The sprite to use (mania-note1T.png) for the tail of slider notes appearing in the first column
NoteImage0T: mania-note1T
The lines that start with // are descriptions of what the line below it does.
Basically you can 'stack' these for each lane/column/key in mania.
So for 5k that would be this.//
Keys: 5
//
//The sprite to use (mania-key1.png) when the key is not pressed
KeyImage0: mania-key1
//The sprite to use (mania-key1D.png) when the key is pressed
KeyImage0D: mania-key1D
//
//The sprite to use (mania-key2.png) when the key is not pressed
KeyImage1: mania-key2
//The sprite to use (mania-key2D.png) when the key is pressed
KeyImage1D: mania-key2D
//
//The sprite to use (mania-key3.png) when the key is not pressed
KeyImage2: mania-key3
//The sprite to use (mania-key3D.png) when the key is pressed
KeyImage2D: mania-key3D
//
//The sprite to use (mania-key4.png) when the key is not pressed
KeyImage3: mania-key4
//The sprite to use (mania-key4D.png) when the key is pressed
KeyImage3D: mania-key4D
//
//The sprite to use (mania-key5.png) when the key is not pressed
KeyImage4: mania-key5
//The sprite to use (mania-key5D.png) when the key is pressed
KeyImage4D: mania-key5D
//
//
//The sprite to use (mania-note1.png) for notes appearing in the first column
NoteImage0: mania-note1
//The sprite to use (mania-note1H.png) for the head of slider notes appearing in the first column
NoteImage0H: mania-note1H
//The sprite to use (mania-note1L.png) for the body of slider notes appearing in the first column
NoteImage0L: mania-note1L
//The sprite to use (mania-note1T.png) for the tail of slider notes appearing in the first column
NoteImage0T: mania-note1T
//
//The sprite to use (mania-note2.png) for notes appearing in the second column
NoteImage1: mania-note2
//The sprite to use (mania-note2H.png) for the head of slider notes appearing in the second column
NoteImage1H: mania-note2H
//The sprite to use (mania-note2L.png) for the body of slider notes appearing in the second column
NoteImage1L: mania-note2L
//The sprite to use (mania-note2T.png) for the tail of slider notes appearing in the second column
NoteImage1T: mania-note2T
//
//The sprite to use (mania-note3.png) for notes appearing in the third column
NoteImage2: mania-note3
//The sprite to use (mania-note3H.png) for the head of slider notes appearing in the third column
NoteImage2H: mania-note3H
//The sprite to use (mania-note3L.png) for the body of slider notes appearing in the third column
NoteImage2L: mania-note3L
//The sprite to use (mania-note3T.png) for the tail of slider notes appearing in the third column
NoteImage2T: mania-note3T
//
//The sprite to use (mania-note4.png) for notes appearing in the fourth column
NoteImage3: mania-note4
//The sprite to use (mania-note4H.png) for the head of slider notes appearing in the fourth column
NoteImage3H: mania-note4H
//The sprite to use (mania-note4L.png) for the body of slider notes appearing in the fourth column
NoteImage3L: mania-note4L
//The sprite to use (mania-note4T.png) for the tail of slider notes appearing in the fourth column
NoteImage3T: mania-note4T
//
//The sprite to use (mania-note5.png) for notes appearing in the fifth column
NoteImage4: mania-note5
//The sprite to use (mania-note5H.png) for the head of slider notes appearing in the fifth column
NoteImage4H: mania-note5H
//The sprite to use (mania-note5L.png) for the body of slider notes appearing in the fifth column
NoteImage4L: mania-note5L
//The sprite to use (mania-note5T.png) for the tail of slider notes appearing in the fifth column
NoteImage4T: mania-note5T
For 7k you would keep 'stacking' this for 2 more keys and put it under the "Key: 7" section, which is under the [Mania] section in the skin.ini.
Then you need to create the according sprites in the colours that you want that match the ones mentioned above.
It gets very confusing very quickly, and requires quite alot of work, but that's how it works from what I understand.