Make his left ark IK-on. Make his right arm IK-off. Parent the broom to his right hand. Select his left hand. Go into the Objects mwnu. In that menu, select "set parent". Set the broom to be parent. That will select his left hand's IK-parent to be the broom. Then pose his right arm, and his left arm will follow it, if it is long enough to reach.

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Don't text-edit extra x/y/zRotate parameter dials in, or you will likely merely make a mess and get Poser's workings in a tangle.