I'm looking for S20 manuals myself, but I can tell you a few things.
The S20 sample rate is 32Khz Max. So 17 Megs gives you 4 minutes sample time. The only way to save/load samples is through Sample Dump Standard (17 Megs is a LONG wait) or through floppy discs (S20 can span several 1.44 Meg discs so you can save the whole 17 Megs). There's also a program called "ADisk" (aka-Akai Disk) that lets a PC read/write wav files to an Akai formatted floppy....look into synthzone's "akai" section for more pointers.
Do a search for "s20" in the discussion group: http://www.antisleep.com/akai/
Lastly, if you do a search with alatavista, there's one pointer that'll come up with the german akai pages. It has (had?) a manual...in german. Akai wants $25 (us) for a copy if you want it in english.