Using await on list joined with db object

Question

I have a linq query as :

IGetAll.cs
public Iqueryable<ClassB> GetAll()
{
 _entityframeworkService.GetAllAsync();
}

Program.cs

List<Class1> listClass = data;//readily available

var result = (from list in listClass
              join _getAll.GetAll()
              select list);
var extractedResult = await result; //Error listClass cannot be awaited

Basically I have a list already (listClass) and want to join with db query object which can awaited (_getAll.GetAll()). How can I use await in this case?

if my listClass is also dbObject I could have used await easily