No, the links in these Googles will pride details
Seedcode has a free calendar template - they provide a guide on how to incorporate it into yours AND how to relate them.
With a lot of work, you could bring all the elements of a starter solution into an existing file--but it's not a trivial operation as you have to bring different parts of the solution in at a time and it's likely that some aspects will be broken in the process and you then have to find and fix such issues.
You can, however, put table occurrences (The boxes found in Manage | Database | Relationships) into the invoices file that refer to tables in your contact manager and you can put table occurrences that refer to tables in your contact manager file into the invoices starter solution. This allows you to add layouts that refer to data in the other file and you can link data from both files in relationships.
This also takes some work to link everything in correctly, relationships, scripts, layouts, etc may all have to be updated to refer to data in the new table occurrrences, but it's much less work than merging both files into a single file system.
I've also heard of a tool called FMMigrator that is supposed to be able to merge two Filemaker Files intoa single file.
Thanks both for your help. I think I'm going to just stick with a starter solution, until I build some more knowledge.
This would be great - has FM been updated since to allow importing starter solution templates for various layouts...?