Most people want to know how to do the opposite as a split file offers distinct advantages when the need arises to deploy an updated copy of your database as it can reduce or eliminate the need for importing data from the original file into the new version.
You can import the tables, but not any relationships defined in that table. But you should have most if not all of those relationships defined in the interface file any way. If so, you can use import records with the New option for specifying a target table to import each table, then you can double click each of your original table occurrences and "redirect" them to newly imported tables instead of an external data source reference to the same table in the other file. But any relationships defined in the data file that do not also exist in the interface file will need to be manually recreated.