You can add other thesaurus files to the configuration by editing the FTR_THESAURUS_LIST line of the site.cfg configuration file (Directa) in your system. For example, to add a thesaurus called 'nouns', create a line like the following.
set FTR_THESAURUS_LIST "c:\\fultemp\\nouns.fth,c:\\fultemp\\verbs.fth;"
A user-created thesaurus file must be compiled before it can be added to the configuration.
For more information see Compiling a Thesaurus File.