iosspellcheck
In iOS 7, one could train spellcheck to recognize acronyms by, e.g., typing 'My Story (MS)' so that when you subsequently typed 'ms' it would appear as 'MS'. This no longer works — or, at best, works occasionally and often forgets.
The iOS 8 advice, e.g., from http://www.wired.com/2014/10/quicktype-tips-ios-8, to select text and double-tap the shift key to get 'all caps' suggestions in QuickType, is tedious and works only occasionally.
How does one train the spellchecker for acronyms in iOS 8?
Just right click in the working tab, select Spelling and uncheck the option "Check Spelling While Typing"
The most prominent app would by TextExpander, available for Mac OS and iOS. Another alternative would be Typinator only Mac OS.
Another option (on iOS 5) would be shortcuts (Settings -> General -> Keyboard). You can add shortcuts and replacements as in your example. This is probably the best solutions as it is free and works in all apps. (TextExpander works only in some, that explicitly support it.)
Best Answer
You can manually add them by using the keyboard shortcuts feature.
Go to Settings > General > Keyboard > Shortcuts then add as many as you want.
It may be a little tedious at the beginning but the settings will stay there forever.