First of all, this concept pertains to the main Shin Megami Tensei series. Remember, we're dealing with spirits. Not manifestations of one's true self.
As for the compendium, we can only document the types of each spirit. (Knockers, Shadow People, Apparitions, etc.) Meaning, we can't save and duplicate spirits, but we can document them for future paranormal encounters.
Take this entry I wrote for example:
Knockers
Knockers are spirits found in mine shafts and common urban households. Knockers in mine shafts would make knocking sounds on the walls of the mine to either inform the miners that there is a precious mineral there, or that the mine is about to cave in. In a common household, Knockers often knock on walls, doors, and other place throughout a house. Knockers are often mistaken for Poltergeists. Knockers can be used to provide a distraction. Send a Knocker to make noise, and they can lure people, animals, or maybe even other spirits to certain areas.