You can create a new keyboard layout from the currently active keyboard layout. That is, whatever keyboard layout is shown as active in the Input menu (the menu with the flag) can be converted to a keyboard layout file that Ukelele can edit. Just choose “New From Current Input Source” from the File menu, and it will create a new keyboard layout and open it for you.

There are some cases where this is not possible. If you have the Unicode Hex Input keyboard layout active, for example, Ukelele will report that it cannot handle complex keyboard layouts with multiplier actions. Some other input methods may not work as well, as Ukelele can only handle keyboard layouts, not general input methods.