Hi,
I've owned Springers for most of my life and have helped to train many more. Currently, I have 5 of them in my house.
What you want is to steady your dog to wing and shot. This is considered a skill level for a finished dog. And mostly taught to dogs that will compete in doggie games like Trials and such. But some people like their dogs steady.
But first there are a couple of questions you need to answer for yourself before attemping this.
1. Are you willing to do the ALL the work needed to steady the dog, and All the work needed to keep the dog steady? There is no "halfway" measures here. You are either willing to do this or not. If not, be happy the way your dog works now and save yourself and your dog a lot of hassel, aggravation, and disappointment. Even if you send your dog out to a Pro for steadying, you will need to work to keep the dog there.
2. If you aren't into the doggie games, then are you, (and anyone else who maybe hunting with you), willing to pass on game in the field if your dog even looks like it is going to "break"? Because even just 1 shot over a steady dog that breaks will send you right back to square one in training. And never work your dog with an unsteady brace-mate.
3. Can you get help in your training? Steadying, generally goes best if you have two helpers to gun for you. (you don't carry a gun, you handle the dog during training).
If you can answer yes to all these questions, then perhaps you may decide to do this. If you can't, then I would strongly suggest you don't. As I said earlier, it will save you and your dog a ton of disappointment and heart-break.
I'm not trying to dissuade you from this, but rather I'm trying to help you make a good choice for you and your dog. It does sound like that you have a well trained field dog. And if you can call the dog off a fly-away with a whistle, then you already have a dog that would be welcomed in any hunt.
If you want I can do another post on the steps that you would need to go through to steady your dog. It would be long one though. Even more long winded than this one

!
Dale