If you're wanting to give your Shas'nel a helmet, then you should know that you're likely going to have to cut off that triangular vent thing on the model's back. It is possible to just about fit a Firewarrior helmet onto the model with the vent in place, or better yet reversed, but there's almost no room for vertical movement.
The Fireblade model's head also comes attached to a chunk of the body, so you'll need to fill in some of the body with greenstuff to make a helmeted head fit.
Aside from that, I've found it's fairly straightforward to alter the pose by altering the legs. The method I used was to cut one leg in half at the knee, then join it together using wire and greenstuff to make a bent leg.
The result looks thus:
Other minor conversions present include replacing the knife hand with a grenade hand from the (4th edition) Firewarrior kit, and adding a pair of data-cables connecting the Shas'nel's markerlight to her helmet using florist wire. A 3rd/4th edition Firewarrior bonding knife was also added to the back.
The raised arm seems like it could be cut off and reposed or replaced simply enough as well, but I never attempted this.