Enable new Office 365 group to email on-premise mail enabled Security group

Hi we have an onpremise top level email security group e.g all_in_ABC

This group only allow certain individuals or groups to email. Its member are also other mail enabled security group e.g all in marketing, all in sales. So we cannot convert these groups to the newer office 365 due to having an onpremise file server.

We have a new office 365 group e.g abccommittee

How can we allow office 365 group abc committee to email on premise all_in_abc because office 365 group have no ldap structure for us to add to the attribute of all_in_abc.

We have only 1 way aadsync without azure ad premium.