NSDecimalNumber を使うといいらしい。
let str1 = "5.5"
let double1 = 0.5
let decimal1 = NSDecimalNumber(string: str1)
let decimal2 = NSDecimalNumber(value: double1)
let decimal3 = decimal1.adding(decimal2)
or
let decimal3 = decimal1.subtracting(decimal2)
or
let decimal3 = decimal1.multiplying(by: decimal2)
or
let decimal3 = decimal1.dividing(by: decimal2)
let double2 = decimal3.doubleValue