Dear anas, ACCA degree holder can appear in css exams, Completion of part 1 and 2 is equivalent to uk bachelors degree and completion of part 3 is equivalant to masters degree in uk.I qualified my acca degree in feb 2007 so this is upto date information.You need to contact ACCA and ask them for "equivalance letter" and explain to them wat actually you want.Its free.
After getting that letter i am quite confident you can appear in css exams, because FPSC does accept international graduation and masters degree.

Best Of Luck