Do you mean you want to enable Skype for Business for this user in hybrid mode?
Based on my research, you could move a user who was previously migrated from on-premises to Teams back to on-premises. To move users from TeamsOnly mode back to an on-premises deployment of Skype for Business Server, use either the Move-CsUser cmdlet or the Skype for Business Server Control Panel. For more details, you could refer to: