You can build the Expression by hand, like so:
var eParam = Expression.Parameter(typeof(Employee), "e");
var comparison = Expression.Lambda(
Expression.LessThan(
Expression.Property(eParam, "Salary"),
Expression.Constant(40000)),
eParam);
return from e in db.Employee.Where(comparison)
select new EmployeeViewModel { Name = e.name, Salary = e.Salary };