How to create Special Stock Partner SB in Business Partner BP

I dont see the option SB in partner function while trying to add special stock partner in partner function of customer master (Tcode BP). Is it possible? and can it be used in SD? I have used partner function in tcode XD01 but this is the first time using tcode BP. can someone please explain me the steps. The special stock partner i m trying to add to orignial customer master is set to SP. Customer master has BP and payer already added to partner function. thanks.