BoltMail does not specifically have a "merge lists" tool however you can use the copy subscribers tool to do the same thing.
If you want to merge subscribers into a new list then first create that new list.
Click on the list you want to merge the subscribers into. This could be a new empty list or an existing list.
Then click on "Tools" and then click on "Copy" and follow the instructions. This will copy the subscribers from the list you choose into the current list.
The copy tool only copies one list at a time so repeat this process until you have copied all the lists you wish to merge into the current list.
BoltMail will not copy subscribers that already exist on the list. To prevent accidental spamming BoltMail will never allow a list to contain duplicate contacts.
To sync differences between lists you can use the "Sync" tool found at Lists > Tools