In theory, from the ribbon you can use the External Data tools to copy "stuff" from one database file to another easily. So there is where you would find the controls to copy stuff from point A to point B. Having said that, ...
Recombining a split database is certainly possible, but pretty much NEVER as efficient if you planned to have multiple users sharing it. The split isn't done for ease of development. It is done to minimize problems caused by file locking and attempting to share something that will be keeping LOTS of file locks open while in use. Reverting to a monolithic database INCREASES the odds that two people who inadvertently try to use the same form on the same record at the same time would crash you with an "unrecognizable database format" error. (Trust me, you REALLY don't want that to happen.)