This is exactly what I have been looking for.

I have 3 different sites, these are online shops in Joomla / Virtuemart.

www.iwarehouse.co.za
www.iwarehouse.co.za/digital
www.iwarehouse.co.za/africa

Before I buy JMS2WIN I need to know if it is possible to "merge" these EXISTING sites into 1 database, but have them keep their existing "look and feel" without having to redesign them from scratch?

My architecture would still be as many databases as their are sites.

Move existing website as JMS slave site is possible and require manual processing.

The idea is to create a master website that has all the extension required by all your websites.
When you have installed all the requested extension, you will be able to create 3 slaves sites corresponding to you exisiting website.
When done, you will have to replace the "fake" DB by the real data of your existing website.
Perhaps you will have to change the table prefix to merge them into a single DB if you used the "jos_" for all your existing websites.

You will also have to copy all the images and all the specific files required by your websites.