Assuming the character is right handed, parent the weapon to right hand. Now parent the left hand to the right hand and turn on IK for the left arm. Now make your poses ONLY by using the right arm. Both the weapon AND the left arm should move with the right arm.

Let me make that clearer. You want both the weapon AND the figure's LEFT hand parented to the figure's right HAND. But you want IK turned on ONLY for the left arm. Get back to me and let me know how that works out for you.