You will not be able to merge your historical data from multiple GA properties. What you can do in the future is use the same exact GA code on multiple sites to 'merge the data'. If you take this route and later want to view your data on a per domain basis: you will either need to write your own custom filters to be able to filter out your data by domain name (you can do this any time), or you should add a custom dimension or variable (have to do this now) so you can tag each pageview with the domain of your choice. I would use custom variables for this, which makes reporting a lot easier.
You might also want to take a different approach.
1. Create a property for each individual site, so that each site has its own reporting in GA.
2. Create a new property called "all sites" and then use that on every site as well.