koji add-host --krb-principal= host will work, but it can't be changed later, like it will be possible for an user. koji edit-host --help don't show an option for it.
Each host has a corresponding user entry, and the principal field is part of the user entry, not the host entry. The add-host command has this option as a convenience, but the edit-host doesn't have a corresponding bit of syntactic sugar.
add-host
edit-host
At the moment, the way to do this is to use the edit-user command. The user entry should have the same name as the host.
edit-user
Metadata Update from @mikem: - Custom field Size adjusted to None
Yes this will also work as an workaround. In first I have done it direct in the database, but this will be an very hot operation.
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/4138
Please continue any further discussion there.