SongSelect only makes chord charts available as PDF files with the integration used within the OnSong app. These files are more like a photograph and contain no text. Because of this, OnSong cannot format songs, detect keys, or perform other OnSong functions. These files are viewed "as-is". However, there are a couple of ways to work around this limitation.
SongSelect Premium users now have access to ChordPro-formatted songs that work great in OnSong. While these files are not available for import directly in the OnSong app, you can view the SongSelect website in Safari to download the ChordPro files and import them into OnSong. This will give you all the power of OnSong including changing the format and transposition. Here's a video that can show you how: Importing ChordPro Files from SongSelect
SongSelect files can be transposed using the Chords tab in the Style Preferences Menu. The transposition is made possible by making calls to the SongSelect web service and returning different PDF files. Because of this, an Internet connection is required the first time you change keys. The transposed PDF file is saved to your OnSong library so you can transpose it into that key again without requiring an Internet connection.
While OnSong has a number of tools for extracting text, it's difficult for OnSong to do this with SongSelect files. If you use the Extract Text feature in the Conversion Toolbar of the Song Editor, OnSong will attempt to scan and perform optical character recognition which can have mixed results. When you import a SongSelect chart, the lyrics are also included so you may find it easier to place chords in the Song Editor and then type off the Use Imported File Instead of OnSong switch within the Conversion Toolbar.