Generate map from existing layout?

There’s no built-in feature for this, but you can write a custom import script and use JavaScript’s Tiled’s new (snapshot/1.5) image parsing capabilities to map from the image to tiles. You’d read the image’s width and height, make a new map document of this size, assign your tileset to it, then iterate through the image pixels and assign the appropriate tile based on the pixel colour for each pixel/cell.

It can be pretty challenging to work out how to actually put the tile data into a Tiled document and the documentation doesn’t provide an example, but hopefully this guide on the forum will help.