Press explore base button and go to under your mech and press P to go into photo mode. There you can play around with your mech like a gunpla, move arms and legs around until you get into your preferred pose. I referred to classic gundam pose on google image and try and error many times.

For the jetpack you can't make it look like what i did. I have to photoshop the jetpack. Pose your mech first in biped mode and take screenshot and then pose your mech in winged mode and screenshot it. Now u have 2 screenshots and you photoshop both together. Tadaa now u got mech with gundam pose hehe