To lock content behind a new SKU, you need to do two things: tell the content what SKU to use, and tell the account server it exists.

Account Server

Edit defs/account/product_catalog.def to add your new SKU:

CatalogItem
	Title "Your Title Here"
	SKU NEWSKU__
	InventoryType PlayerInventory
	InventoryCount 1
	GrantLimit 1
	Global 1
	Recipe NEWSKU__
End

Replace NEWSKU__ with your own eight digit SKU.

Costume Pieces

Add a product line inside the mask or info block telling the game what SKU locks this costume piece:

Old:

Mask
	DisplayName "Chisel"
	Name "!boot_chisel"
End

New:

Mask
	DisplayName "Chisel"
	Name "!boot_chisel"
	Product newsku__
End