Hi,
i’ve read the manual and understand that renoise can sample sounds from midi-devices.
I like the drumkits and sounds of my MC-303. Is there a efficient way to sample ALL instruments and drumkits to renoise? If possible the instruments also as multisample?
Just connect the MC-303’s output to your line-in, press record in the sample editor and you’re good to go. There isn’t really an efficient way to sample all sounds at once so you’ll have to sample them one by one.
Have fun! 
I’ve done the same kind of thing before both with hardware and sample CDs (before they came with WAVs!);although not with Renoise)
I created a midi file just with individual hits ie C2 C#2 D2 etc with plenty of space between each hit. I played the midifile and recorded the output into Wavelab and started chopping from there (IIRC, I used ReCycle as well to help speed up the process). You don’t have to use Wavelab etc, just any audio editor that you’re comfortable with
It’s a ‘No go’ for multisamples I’m afraid cos there aren’t any! ie the MC-303 does not contain multi-sampled sounds. You could sample the sounds at different velocities but it’d be pointless because you can set the velocities in Renoise
I’d try to make up a basic drum kit first ie one octave and see how you go from there
Good luck!
(I loved my MC303; might have to dig it out and see what’s on it!)