Seems like you are trying to assign a different approver when an existing approver is inactive in sys_user table. If yes, then out of the box there is no such solution which means you have to customize it. So, if the approver for such RITM is set based upon a workflow or any other server side logic then you need to tweak it to send approval to the inactive user's manager (an example): https://www.servicenow.com/community/developer-forum/when-user-inactive-approval-send-to-manager-for...
