Donjon maps

Is there a way to just import a randomly generated map from donjon into roll 20?

Not with one click, but yes. After creating the random map, you can either right click and "Save image as..." or use the "Player's Map" and "Print Scale Map" as needed. From there, you upload it into Roll20 as an image and place it on the map layer. Then comes the fun of stretching and skewing the image until it fits. The wiki has a good section on aligning maps that will probably help with this. I don't typically use the Donjon map as it, but instead use it as a guide to place tiles on top of.

You might find some useful wiki info here as well. Useful links for mapping