how do i setup the main store with multi stores.

Hi guys, i super newbie and my goal is to have a
"main store" ex. http://www.mystore.com and that store will have about 400 categoreis.
what have i done: i have made my server work suceffuly with subdomain and mulitstore working i have also installed marketpress, Pay To Blog, Post Indexer and theme FrameMarket.

so my idea is when a customer signup for a "new1 sub store" ex. new1.mystore.com they will have all the categories there admin so they can add there products and the products will be seen on my main store. is this possible?

Warm regards Erik