Hi i am also trying to achieve this functionality as i already have my player prefabs with other scripts attached already but have been referencing materials and textures individually through unity. My single player prefab is up to 120 draw calls in itself and this is going to be no good for a mobile platform.
I would also like to add a 2dtk sprite to my existing planed character prefab. My prefabs fbx consists of 6 planes, arms, legs, chest and head.
when i go to add the 2dtk script to my plane with my characters chest texture for example...

This is probably because it has not been added as a 2dTK component yet like you said.
I am very new to this. How do i go about using the script you provided to Add my plane as a 2dTK component? Ultimately i would like to create a script to attach to my individual planes that adds it as a 2dTK option to begin with and not have to worry about it again.
I could technically workaround this by completely generating all new planes with GameObject>Create Other>tk2d>Sprite and sizing them accordingly to what mine are sized at and then overwrite my prefab but this seems like a whole lot of wasted time and probably not necessary.