new query verification in sql.conf

I use freeradius for authentication of pppoe wifi. I need to make new sql query in a table with a list of mac-address, if the CallingStationId will be equal to the some mac-adress of the table then will not have to be connected. A system of mac-adress blacklist. I tried to make adding one query in sql.conf but it does not function. I find that it would have to add a new function in rlm_sql.c, but am not habil C programmer. Somebody can help me? Sorry for my english Thanks Jean