I started making music on a C64 in the 90s, sold mine nearly ten years ago but have been playing around with some editors in emus lately and I actually plan to buy me a hardware C64 again soon, to use it as a synth in my setup.
Some proper answers:
Samples: They are theoretically possible but for some reason not much people used them after the 80s. The reasons: The C64 is slooooow (less than 1 Mhz) and playing a sample took alot of processing power. Most of the tunes you know that are using the digichannel are from the loaderscreens from games or games that had not much happening on the screen.
Also there are two generations of SID-Chips and the newer one had a problem playing those samples, so people just ignored samples later.
SID2MOD: I highly doubt it because you can’t compare a SID-File to a MOD-File.
A MOD-File is a container that hold the pattern-data and the samples and that needs a seperate player.
A SID-File is basically an assembler subroutine/program that needs to be executed regularly (normally 50 times per second) and which starts to make sound.
Altough you make your music in a soundeditor that definately has similarities to a tracker you need to compile your music before you can actually play it or before it can be converted to a SID-File.
So you made your music in the soundeditor, saved it in the native format of the editor and when you where finished you compiled it. This was a one-way-street, few of the editors I know had a “decompile” option, so it was not possible to load compiled music from other people.
Ofcourse the native format of this soundeditor might be comparable to a MOD-File, but I highly doubt that someone wrote a converter for one of the many soundeditors avaiable for the C64.
However, that being said, I think someone made an attempt of a SID2MID some time ago which was based on Frequency-detection, but I think he didn’t got very far.
Making your own: I really don’t want to discourage you, but if you plan to make music on the C64 then prepare to spend a few weeks reading manuals. Making music on that thing is 50% programming in something very close to assembler, there are no neat GUIs or similar, you will have to modify bytes of the SID-Chip directly using tables, and you really need to be fit in hex.
However, I think I saw a specific “C64 Soundprogramming” Manual somewhere that concentrated completely on the SID-Chip, but that one had a few errors in it aswell.
Also, the editors are mostly terrible when it comes to usability, there is no “undo” or other nifty things that you take for granted now.
However, if you really want to dive into this then I can recommend the DMC5.2 because there is some half-decent documentation avaiable for this one.
And if you just want the sound of the SID-Chip : Start saving alot of money and try to get a sidstation.
For a short overview of what you want to get into: