If you really need that much insulation for your legs, I'd personally recommend just getting some insulated pants, 3/4 pants, or shorts. It'll make your system a lot more versatile if you ever ski when it's warm out.
There are now a lot of options for insulated pants/shorts, but I've linked a few below. The cheapest option would just be to layer some heavy fleece sweatpants under the shell. If you're super cold, look for something with Primaloft or some other synthetic insulation.
*also The North Face is coming out with a 3/4 pant version of their Ventrix jacket, which should be pretty sweet and not crazy expensive.