You can load styles via the different styles panels from an existing menu.
Or it might be eaier to create a text with every style existing and objects with every style and tables and cells. Save this as an itme in a library.
Then you can delete all styles and drag the item into that document. Be sure, that none of the styles is based on the default styles, which might cause problems with documents started from different versions of InDesign.
I am sure it is scriptable, but I am not very gifted in scripting.
Change the file extensions temporarily to .indd, add them to the book, synch, then change them back to .indt.
Essentially all the extension does is tell ID how to handle the default opening procedure. It has no effect on the file content.
Cool! That will actually be useful to know in general. Is it only .indd and .indt files that are identical?
What other types would you expect to be the same except for extension?