Hi Sabine,
Unfortunately, this is not yet supported in Skyline. My apologies. We certainly know it is an interesting case. Just haven't managed to implement anything for it yet.

Some people have done some interesting hacks with single amino acids and chemical modifications on them. So, it is conceivably possible that you could define a modification on the y1 or b1 ion that would get your your desired product m/z.

Sorry I don't have a better answer yet. Look for better support in a future release.

--Brendan