Any cube has a source database, quite often that database is different from original OLTP source, but it is still a relational database. It means you can generate an entity model from an existing relational database by using existing tools for ADO.NET Entity Framework. Just make sure that you generate your entity model from the same database schema, that is used as a cube source. The process of generating an entity model from a relational database is well documented in ADO.NET Entity Framework documentation.
You do not have to generate your entity model from a relational database - you can also use Entity Framework Code First.