You can subclass dbfunction call and override ToQueryText, you can also use a trick:
employee.Fields[(int)EmployeeFieldIndex.Salary].SetExression( new DBFunctionCall("current_timestamp{0}",
new object[] {string.Empty}"));
this will trick the dbfunctioncall the function is pre-formatted, so it won't append () itself, but there's no preformatting going on, you append an empty string