Backup and restore SQL server database with FILESTREAM filegroup
I would like to know if is it possible to modify the content of all filestream columns when performing a backup


No.

I would create my test database with the same schema as the production database, and then write a custom script to sync the data in the primary filegroup (this is fairly easy as your production system is the source, so the sync is only in one direction).

Then with the filestream data, I would stub the files, which could be done any number of ways. Off the top of my head, I'd somehow generate zero-byte files on the test side.