At the moment, there is no way to add your own watermark. However, if you have a png file for your logo, you can go in freestyle mode and add it as a photo. Once you do that, long tap on your photo and call out the context menu. Tap Collect. Now you can access that "logo" at the end of the bottom row under Collected whenever you want as a watermark.
Additionally, you can also opt to add text to your collage without a background to give it the appearance and style of a watermark.