Currently, there is no way to do this. The best option is to place the tiles in your tilesheet such that they’re all along a uniform grid.
You can create an Image Collection where each tile uses a different subset of sheet.png, or uses the original source images for the tiles. In the future, there will probably be a new tileset type that will let you define tiles from arbitrary rectangles in an image (issue #2863).
Keep in mind, however, that there’s no per-tile drawing offset in Tiled, so some of your tiles will still not be properly usable in Tiled because they’ll be anchored to the wrong part of the tile cell.