I can't speak for the iOS version but I know you can't on Android.
If you want to use a specific font and you have Photoshop or Photoshop Elements (or other image editor that can read PSD files), export from PS Touch as a PSD and open it in your image editor to complete the process.
I was disappointed that I couldn't find an answer to this but I think I've got it.
Try installing HiFont from the App Market. It installs fonts on your android that work across most apps. Open HiFont, slide over to the Local tab. It tells you how to install new fonts to the program. Just make sure the font you download is a .ttf file.