Try to use RealDigits:
RealDigits[fp, 2, 8, -1][[1]]
this gives a list of numbers, you could use ToString and StringJoin to make a complete string.
here is a working example:
GiveMeRealDigitsBehindCommaString[number_,base_,length_]:="0."<>StringJoin[ToString/@RealDigits[FractionalPart[number],base,length,-1][[1]]]
GiveMeRealDigitsBehindCommaString[N[\[Pi]],2,8]
this will return:
"0.00100100" (as a string)
