[Openstack][nova] server groups

Hello, we noted that instances can be inserted in server groups only at instance creation step but we need to insert in a server group some old instances. We tried to modify database nova_api server group tables but we noted that we must modify spec in request_specs table . For us is not clear how to modify the spec value. We tried to investigate looking at instances inserted in a server group at creation step and we got issues in instance live migration. Please, anyone could provide any utility to do it or any template ? Thanks Ignazio