There is no way to migrate a Yammer network from one to a different tenant. You will always get a new Yammer network. The statistics you see in the network merge wizard should be correct. You can also export the Yammer statistics to CSV files, which will include stats for all the networks/domains in your tenant (O365 central admin -> reports -> usage -> Yammer activity). I use these reports and add pivot tables to see how active the networks are before merging them.
You can create an account on these networks by adding a proxy address to your existing account. The only way to get (verified) admin access to these networks is by creating a global admin account in O365 with an email address for that specific domain (proxy address only gives you a normal account). Creating an admin account allows you to post an announcement in the all company group to warn users that they have to create a backup as the network will be lost.
You can't automate the merge for future acquisitions, unfortunately. It's best to merge new networks into your existing network as soon as an acquisition takes place to prevent users from signing up for accounts on these networks.