To make a model hold something, you can parent it to the hand. Select the prop in question and use the command "Set parent" in the properties. This allows you to select a body part (e.g. the right hand) of the model to parent the prop to. After that, it will follow the movement of the model in posing and in animation. Some props are already parented when you load them, if the creator of the prop has chosen to do so.