That's not currently possible. In 'Database -> Purge Hands -> Advanced' there is an 'Observed Only' option however there is an issue where that purges all hands not played by the active player rather than just observed hands so I'd recommend importing the hands you want keep into a new database. If you do not have all the original hand history files then you can export them from the current database
and don't forget to export your player/hand notes if you have them (Database -> Notes). Also if you have a number of tournaments with manually entered results then exporting those tournaments will create summary files containing the manual results so they can be imported into a new database.