Note that you can encode hand made (or other externally generated) puzzles in a format that Simon Tanthams app accepts, and share them.
I'm not aware of anyone actually doing this for most puzzles, but the crossword world often lets you download puzzles in a generic format to play in different apps, and the apps often have ways of regularly downloading new puzzles from the web.
I believe there's also apps that can prefill a soduko grid from a photograph of a grid, so maybe a translator could be built to pass those to the Puzzles app in the appropriate format.
I'm not aware of anyone actually doing this for most puzzles, but the crossword world often lets you download puzzles in a generic format to play in different apps, and the apps often have ways of regularly downloading new puzzles from the web.
I believe there's also apps that can prefill a soduko grid from a photograph of a grid, so maybe a translator could be built to pass those to the Puzzles app in the appropriate format.