Hi Kenny,
Odata is an open standard, not a proprietary solution from SAP.
Odata supports $expand: URI Conventions (OData Version 2.0) · OData - the Best Way to REST
If you are referring to Odata in SAP gateway, $expand is supported in SAP gateway by default. You could have a look at this: OData Query Options - SAP NetWeaver Gateway - SAP Library
Regards, Midhun
SAP Technology RIG