Apparently, Seemaab's brother Shadaab is a close friend of Sajid's and recommended him. While Sajid, who has immense respect for late Amjad Khan and regrets the fact that he never got the opportunity to work with his childhood idol, was more than happy to have Seemaab on board.
Incidentally the original Himmatwala featured Amjad Khan playing the role of Sher Singh Bandookwala-the role now being essayed by Mahesh Manjrekar in the remake.