Substitution : Payment block (bseg-zlspr)

Hi,

I would like to substitute the payment block when posting document header (tcode:FBR2). However, I cant find the BSEG-ZLSPR field to be substitute in the substitutable fields (tcode:ggb1 Financial Accounting > Document Header > Substitution > Step).

Is that means that if the BSEG-ZLSPR doesnt appear in the substitutable fields to be selected, I CANT perform any substitution to that field?

Appreciate advice. Thanks.