Found out this way, which is working.
xaml:
<ribbon:RibbonComboBox Name="cmbBoxComPortNameRibbon" SelectionBoxWidth="40" Label="Port name:">
<ribbon:RibbonGallery Name="galleryComPortName" SelectionChanged="GalleryComPortNameSelectionChanged" SelectedValuePath="Content">
<ribbon:RibbonGalleryCategory Name="galleryCategoryComPortName"/>
</ribbon:RibbonGallery>
</ribbon:RibbonComboBox>
code-behind C#:
galleryCategoryComPortName.ItemsSource = SerialPortAdapter.GetAvailablePortList();
SerialPortAdapter code:
public static ObservableCollection<RibbonGalleryItem> GetAvailablePortList()
{
var portList = SerialPort.GetPortNames().ToList();
portList.Sort();
var portObsCol = new ObservableCollection<RibbonGalleryItem>();
foreach (var rgi in portList.Select(port => new RibbonGalleryItem() { Content = port, Name = port }))
{
portObsCol.Add(rgi);
}
return portObsCol;
}