Unfortunately, there is no easy way.
You can import the tables into one file. You can create new layouts in the one file and then use Select All to copy and paste all the objects on the layout into the new layout.
You will have to repoint your fields to the new table occurence.
Wow! What a pain! I can't understand why Filemaker did not do the work for us on the Business Productivity Kit BPK. They have 6 different databases making up the solution.
There are options that reduce the pain a bit. There's a product available called FMMigrator. I haven't used it myself, but I'd be looking into it if I had a major system I wanted to merge.
Using just filemaker ( or filemaker advanced would be much better)
You can use import records with the new table option to pull in tables with both data and field definitions. Calculation fields that refer to other tables/files may be imported enclosed in /* comment */ symbols until you re-create the referenced tables and then edit each such definition.
Then you have to rebuild relationships by hand as you can't copy and paste them.
Next reproduce all your value list. You can copy and paste custom values, but other types take more work.
Check the scripts you want to import over for duplicate script names and change them to unique names.
Now you can replicate your layouts by copy and pasting to layouts of the same exact name that refer to table occurrences of exactly the same name.
Check the scripts you want to import over for duplicate script names and change them to unique names. Now import all your scripts.
Go back to all your layouts that contain buttons, delete all the objects you pasted earlier and re-copy and paste them.
Doing scripts and layouts in this order avoids having to find each button and manually re-connect it to the proper script.
Yes it's a pain and often not worth the trouble.
What do you hope to gain by merging the BP files?
Thought was that when I convert to another online database it would be easier if all the tables were in one database. ???
Also though that the application might run faster in filemaker if it is not looking around for the tables, probably not an issue though.