Add a s-tore to a already eisting blog, with different template. Necesito agregar una tienda electrónica a un blog que ya existe, con un templete diferente

The page is [login to view URL] is a mexican site and we need to add a store to the site, that will use a different template wich we aloready have, it should be accesible through the main site's menu. Need help deciding if it yould be a subdomain or jst one more foder. We need to use the main sites look of the new template.