Converting two standalone WP sites into a MultiSite configuration

My client has an existing site which needs to be completely rebuilt ( and also wants another site built using the content from a page ( currently within the existing site that allows users to register and speak to each other either as a community or privately. I’m going to use the Spirit theme to create both. On the new website, I’ll be using the BuddyPress and Membership plug-ins.

I’ll be creating them using Instant WP which is a standalone application that will allow me to show the work to my client, test, and make modifications prior to publication. Unfortunately Instant WP does not support MultiSite. So my question is this….Once I create both sites individually and obtain approval to place them into production can I create a MultiSite and upload the files? The client’s host is Hostgator.