I don't think you can add a window sill without causing damage.

What I would try instead is a shelf or ledge that sticks to the window itself. There are many varieties of plant shelves that have suction cups that cling to windows. Or look at some cat ledges that adhere to your window.

Other cat ledges hook on to the lower window frame, and would allow you to still open/close any window treatments you might have. Something like this: cat ledge

The cover comes off, and you can paint the underlying MDF (or whatever it is) to match your window trim.

If you are mostly looking for space for plants, you can also hang plants from the top of the window frame, or install a very sturdy curtain rod and hand a few plants from that--but watch the weight if you try this. When you water plants, the pots get heavy, so only a few plants and smaller ones.

But if you Google "plant window shelves," you will find a lot of options. I like this one from Etsy: hanging plant shelves

Of course, with something stuck to the window, you have to make sure that you can open/close blinds, shades or curtains, if you need privacy on that window.

You could also consider a plant stand like this and place it in front of your window if it is the correct height.