I would spontaneously do it in steps:
public static Expression<Func<Model, ViewModel>> ToViewModel
{
get
{
return x => GetViewModel(x);
}
}
public ViewModel GetViewModel(Model x)
{
var vm = new PositionViewModel
{
Id = x.Id,
Name = x.Name
};
vm.Employees = x.Employees.Select(p => new Employee
{
Id = p.Id,
Name = p.Name,
Position = vm
}).ToList();
return vm;
}
This way you can still wrap this up as an expression tree.