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I Found the solution myself.
How to recover any app you have only on your iPod (and not in iTunes) and you can not get in iTunes Store:

Preparation 1:
==> make sure in iTunes automatic sync for...

different situation, same question:
I updated on my Mac in iTunes to iReal-b 4.5 (silly me!)
On my first-gen iPod I am still on iReal-b 4.4 - so its working there.
But how can I backup this...