$expand query option (C# Linq)

Using the CRM 2015 SDK, with an OrganizationServiceProxy, how does one add an $expand query option? The Entity class has a GetRelatedEntity method, which returns null, because the Entity classes for relationships are not added. In the older versions (using WCF data services), you could use AddQueryOption("$expand") to accomplish it. This doesn't seem to be available on Microsoft.Xrm.Sdk.Linq.Query.