Ok, there was some confusion about how to resample multiple pads to one single pad, kind of mixdown of a whole song to a pad (NOT talking about Loop Capture). This is valid for at least the 404 and the 555.

This is how it works.
In the manual it is not explained at all, and there is also a mistake in the description of the Resample process generally, which is the solution at the same time. On page 24 under 'Resample', point 11, they say: 'Press the pad TO WHICH you want to resample'. Wrong. At this point the user has to press the single pad OR multiple pads he wants to resample from, the SOURCE(es), NOT the destination. Just the little word 'to' makes the difference. Without that it makes perfectly sense, apart from resampling multiple pads, which isn't mentioned at all in the whole manual.

That's the answer, maybe also for other people who tried to do the same thing.