Can I use an expedition parka for backcountry touring?

Yes, but with important caveats. Expedition parkas are best for backcountry touring in consistently cold climates where temperatures stay well below freezing even during uphill travel. In moderate temperatures (-5°C to -10°C), the heavy insulation will cause you to overheat during the skin up, even with pit zips open. For most backcountry touring, a shell-plus-midlayer system is more versatile because you can shed the mid-layer on the uphill and add it for the descent. If you do tour in an expedition parka, look for models with the longest pit zips possible and body-mapped insulation that's lighter in the sleeves and underarms.