For simple databases, combining files can be done by hand pretty easily.
First print the original file's field definitions, sorted by creation order.
Next create a new table with all the same fields as the original. It's best if your names are all spelled exactly the same, and no fields are omitted or added. Add the calc definitions last, as these depend on other fields' existence. If you have the ability to print to PDF, you could print those field definitions to PDF, then copy and paste them into the new table definition.
With the fields created, you can now redefine the external references to the old file to refer to the new internal table. It may be better to do this step before the calc definitions are entered, as some calcs refer to other tables.
Next import the navigation scripts from the old file. Some of these may not be necessary in a single file solution, but it will make the layout import easier to have them to start. These scripts will need their layouts reset after the layouts are imported.
Now for the layouts. You'll need to create a blank layout for each layout you need from the old file. Be sure to set the correct table occurence as the base for the layouts. It may help the other scripts if the names are the same as they were before. With the layouts created, copy the layout contents from the old file and paste them into the new layouts.
Import any other scripts that are needed, and check that fields and layouts were mapped correctly.
Check the layouts and make sure the fields and scripts were mapped correctly. Some will need to be reset manually.
I've found that the field mapping may have problems if there have been fields deleted in the life of the original file. If you can figure out which fields have been deleted, you can insert dummy fields in the new table to take their place (this is because fields are matched by an internal ID in layouts and scripts.)
If all that's too complicated and you don't mind spending a few bucks, try FM Migrator or FM Robot. These third-party tools can be used to automate some of this monkey-business.