1: Reloading Bolter:
This is fairly easy. Cut the clip off the bolter. Glue the bolter to the marine's hand, but glue the clip in the other hand, so it looks like he's loading his gun.
2: Chucking grenade: Glue the weapon on as normal, but cut off the hand that holds the end of the gun. Glue on the arm that holds the end so his arm is raised. Then, glue the hand on the arm so the fingers are pointing upward. Then, glue a frag grenade or melta bomb to the hand, so he is throwing the grenade.
3: Assault attachments: Glue a dagger, a combat knife or chainsword to the end of a bolter, on the bottom. Then, cut one leg off at the knee and position it as if it were running, and about to stab something with the bolter.