Filtered select with FormBuilder

Hi all: Imagine 3 tables A, B and C. There is a 1-N relationship between A and B, and another 1-N between B-C. So by inheritance there´s a virtual relationship between A and C. So, what I need it´s a form for editing and adding new items to C, using a combo for selecting the field B. But I know the value of A, although this it´s not a field for C, what I mean it´s a combo for B but filtered, like a hierselect but I don´t want to show 2 combo boxes, because the value of the first one is known and fixed. Maybe I´m not explaining very well, it´s more simply that sounds. Well, I´m using DB_DataObject_FormBuilder, and I don´t know how to get that filtered select item directly. Can anyone help me please? Thank you in advance. Regards, Jose Manuel Zea