I don't know of any function to do it, but I think you can do it quite simply by splitting the hex string and passing each part through Convert.ToInt32():
int part1 = Convert.ToInt32("E9", 16)
int part2 = Convert.ToInt32("4C827CEB", 16) //the last 4 bytes
long result = part1 * 4294967296 + part2 //4294967296 being 2^32. Result = 1002011000043