Hi there,
There are some software products that allow you to design jewelry. They're 3-D design imaging products, and some are set up to be especially helpful with jewelry design.
There is one called Rhino Software, for example. There's another - very pricey - package by ArtCam, which actually allows you to design jewelry and then send the tooling instructions to a wax-carving machine to create the wax model for casting.
Either way, if you want to make jewelry with original cast pieces, you'd need to find a company to do the casting for you.
As for assembly of the jewelry, it is possible to hire artisans to do that for you as regular employees. It's also possible to have the work done in another country, like China, if you're going to order very large batches of jewelry - but quality usually suffers in that case.
I would recommend that you hop over to the Jewelry Making forum and post your question there, too. There may be someone with more specific experience in this area who can help you out:
http://www.bellaonline.com/forum/ultimatebb.cgi?ubb=forum;f=207 Thanks for posting, and good luck with your designs!