create bp with role internet user based on already existing su01 users

Dear,

I know there's a function: CRM_ISA_UA_IUSER_CREATE which creates a new su01 user and link it to a bp, now my question is.. is there a fm/class that does the the same but instead of creating a new user, an existing user su01 user is used.

if you create in sapgui a bp with internetuser role you can fill in an existing user. and it will take over: password and roles.

I want write a program that can make bp with internetuser role in mass.

Thank you very much.

Bjorn