Breeze does not officially support WCF RIA Services at this time. However, if you have an Entity Framework domain model backing RIA Services, you can expose that to Breeze through WebAPI quite easily. For an example, see TodosController.cs in the Breeze Todo sample app or reference this page:
Breeze also supports OData backends, so you could enable OData in WCF RIA Services and connect to that. However, saving through OData is not yet supported in Breeze and I don't know how much of the OData spec is implemented in RIA Services.
What type of application are you considering for Breeze? Will it be replacing or running side-by-side with your Silverlight app?