You can add media files from other apps by following these steps:
Step 1: Locate the video in Dropbox.
Step 2: Select a media file and tap on the "drop down menu" at the top right corner to export it, as shown below:
Step 3: Once the file has finished downloading, tap the Open In button
Step 4: Tap the "Copy to PlayerXtreme" icon.
Step 5: The video will be added to the inbox folder in your PlayerXtreme library.