You want to add some, but not all, files in a given directory to the Tripwire database.
Mark a directory hierarchy for inclusion:
/etc -> rule
Immediately after, mark some files to be excluded:
You can exclude a subdirectory too:
The exclamation mark (!) prevents the given file or subdirectory from being added to Tripwire's database.
The Tripwire manual has detailed documentation on the policy file format.