Hi Silver,
The requirement seems pretty easy in BRF. While designing BRF branch to identify next dunning step, You must be using field AUSDT from table FKKMAKO. (Fortunately, we receive whole dunning group in FKKMAKO structure while BRF is being executed.) Now here instead of using AUSDT to check for the gap between next duning step, use field FKKMAKO-MDRKD.
Just check if field FKKMAKO-MDRKD is not blank and dunning date is print date + x number of days and proceed with your current logic.
Hope it helps.
Ravi