I did these a long time ago. The inspiration came from the wonderful Armies of the Peninsular Wars book (which can be found in my bookshelf that should be at the bottom of the page - although I've reproduced the relevant plate and text as part of this post). From memory the conversion was just trimming the peak off the cap, adding a cartridge pouch and leaving the rest up to paint.
These are the original Airfix French Foreign Legion – a great robust set.
And finally the inspiration.