That's not trivial. The document version number is internally done by jackrabbit core, and can not be directly change it. But you can translate to your own codification if you want. The most simply way to make translation is on Servlet.
I suggest take a look at GWTUtil.java ( GWTDocument to Document and Document to GWTDocument ) and then DownloadServlet and ConversionServlet where version is passed as parameter and used to downloading some document version.