I've looked into this quite alot recently and I think this is the position:
You can add an IR to your PPL by studying and taking either the IR exams, of which there are 7 (through Atlantic Flight Training or GTS) or by taking the full 14 ATPL exams. You then do the 50 or 55 hour flying course, depending on whether you want a single engine or multi IR.
If having gone this route you decide you want a CPL then, unless you took the full 14 ATPL exams you will have to take the CPL exams, of which there are 9.
So, only take the IR exams if you're sure you won't later want a CPL as well! The exams last for 36 months from the date of the last pass.
Once you have a JAA IR you can convert it to an FAA IR by applying for a Restricted FAA PPL and you can then convert your JAA IR to an FAA IR by taking a cut down version of the full FAA IR exam (I think its called the Instrument Foreign Pilot exam). Then providing you maintain validity on your JAA IR the FAA IR remains valid on the back of it.
You can go the FAA -> JAA route but have to take all the JAA exams and do 15 hours flying. I doubt it is any cheaper taking this route, even though the FAA IR requires only 40 hours and you can count the time spent elsewhere instrument flying, which means that you can count your 15 hours on the IMC rating towards the 40, which you can't do under JAA!! Bizzar!!