こんな感じですかね。
lang
1//: Playground - noun: a place where people can play
2
3import UIKit
4
5let fullString = "QWERTアイウエオ01234"
6
7var halfString = NSMutableString(string: fullString) as CFMutableString
8CFStringTransform(halfString, nil, kCFStringTransformFullwidthHalfwidth, false)
9
10let result = halfString as String
11print(result)
半角 -> 全角としたいなら、
lang
1CFStringTransform(xxxxxx, nil, kCFStringTransformFullwidthHalfwidth, true)
CFMutableString Reference