There is no option in the ISPConfig GUI for that. In the database, client table, set the parent_client_id for this client to the client_id of the reseller. Then change the sys_groupid of the client record in client table to the groupid of the reseller and in the sys_user table, edit the reseller user and add the groupid of the client in the groups field, separated by comma.
Sorry for bumping an old thread . Did you implement a way to move clients to resellers in some other way than editing db manually?