Is in Charge of the Halo CE3 Online Expo and the Host!
You can but it requires new animations. yiou might be able to get the marine and Spartan animations to join together, but i suges you make new animations for it. so like you could script it as. Press Q to lower your weapon.
So No you can't realy as of right now. but with a little bit of HEK knowledge you shouild be able too.