I am unsure how to, or if I can.
You can import images from Camera Roll into a layer and fill a selection or layer right from a picture taken with the camera while working in Photoshop Touch. These are two separate things though.
- To add a photo, use the icon with the picture with the plus symbol in the upper right-hand corner. The icon appears in the upper left when working on a picture or bottom center when starting a new project
- The Camera Fill option is the last one under the Amperstand "&" menu in the upper right when working in a project, this option will fill the contents of the currently selected layer or selection on that layer.
A good tutorial for getting a sense of how the Camera Fill option works is the "Create Camera layers" one. It has a picture of a mans head with a pirate outfit. This tutorial walks you through how to use the camera on the device to replace the original man with a photo of yourself.
Hope this helps,