Is it possible to add screen captures from external sources such as Snagit?

Yes, although both method aren’t exactly time efficient.

1. You can use the copy and paste function in the image editor you’ve assigned in your UPK developer settings. Opening the screenshot you wish to insert then copying it and from within UPK’s topic editor click the edit screenshot button and paste it into the screenshot.

Or

2. Create an overlay using powerpoint. Click the Create Overlay button and once Power Point opens insert the screen shot into the slide and “send it to the back”