Not directly pertinent, but my RST and Nomad are COMPLETELY different bikes. The geometry changes result in radically different personalities, and I expect that the Sherpa is quite a different ride that a Nomad by extension.
I've got a pile of parts, enough to build two 700cc derailleur frames up, but after living with a Rolloff, I don't think I can go back to slumming it, so I encourage the change-over. My experience in the past has led me to vow to not compromise, so in your shoes, I'd not try to adapt the sherpa. Live is shorter that we always think, and we can't take it with us ... so spend the money, do it right, and get a Nomad - or a Raven, if you can live with less that 60 pounds on the bike. I find that the Nomad is really at home with very high loads only. For the 45-50 pounds that I tend to lug around, the RST seems to be much more lively and fun.