Yes, window treatments can be added to the designs. We have an existing video that demonstrates the steps on how to add window treatments to your design boards.
Watch this video and skip ahead to the 8:30 minute mark to see how curtains are added:
If you are recommending custom fabric for a curtain panel, you might want to consider creating a simple 2D Moodboard design where you can include an image that shows the style of the curtain panel.
Adding a Fabric Swatch next to the panel to indicate the recommended fabric also helps visually.
Lastly, you can use the Text Tool to add notes to the board and include an arrow in the design to make this clearer for your client.