You can go to Regular Cast and delete the actor. You can then proceed to add him per episode by going to each episode he participated in and adding him as a guest star.

You should also read the following from the Contribution Biblie.

The regular cast section is only for the main or starring cast of the series. It is reserved exclusively for cast members with a series regular status per a season on-screen credits.

Any actor not credited as a series regular—including but not limited to recurring guest stars, guest stars and co-stars—should be added as a Guest Star instead. These actors not credited as series regulars might appear in one, a few, half, most, or even every episodes of a season.

Preferably, only the series regulars who are in all or most episodes of a season should be added as season regulars. Series regular actors are generally credited in every episode of a season, but the characters might not appear in every episode. It is completely normal for regular cast members to be missing from one or a few episodes of a season (i.e. credit only). This is often the case with television series with large ensemble cast. However, if a series regular is only in half of a season, they can be added as a guest star instead.