Print dialog and custom values

Hi all. In print dialogs in Gtk/Gnome applications, as well as LibreOffice (and even MS Windows), I’m able to enter custom values in editable fields, for example:

But in KDE/Qt, I can’t do so. The fields are not editable:

Here, I can only select a value from the combobox, but it is not editable. Is this a bug? Or is it me and did I miss something?