As I recall there was NUBUS raid cards out when NUBUS was the current card standard. Now however,
1. Finding such cards might be hard, perhaps ebay or those vendors who sell old mac stuff. macresQ and others. You will also need a nubus adaptor as well, an other outdated item to come buy.
2. Why bother?? however you cut a 6100, it will be slow. 30 MHz bus slow CPU slow everyting...
3. If you just need a file server you are better of with a 7200 with a PCI IDE card and a second hand 10-20 GB HD, I think it will not cost you more than upgrading the 6100.