Hi,
You can try combining the window with an exposed lintel component (a box). See an example of how to combine windows, and turn them into a new wall cutting component here:
How to Combine Flex Components – FlexTools
You will also want to ‘exclude’ the lintel from affecting the opening in the wall. More on excluding here:
Learn WallCutter – FlexTools
As long as the lintel does not extend beyond the window’s boundaries, on the left and on the right, you should be able to scale it freely horizontally. Vertically it will get distorted.
I hope this helps.