SetPose makes it easy to add various props to your poses. In the handheld prop selection menu, you can choose from props like a sword, knife, balls, a tennis racket, and more.
In order to pick a prop, first, open the settings panel if your on mobile, or go to the right-hand panel if your using the desktop version. There you'll find the model selector, which will say "Basic model" by default. Next to that is the button with a sword icon to open the prop selection menu.

After you've opened the prop selection model, you can select a prop from a list of option, for both the left hand and right hand.

The handheld props will stay in the model's hand, whichever way you move them.

Some preset poses are meant be combined with handheld props. If the button includes the "i" icon, hover over it to see what prop should be used in which hand.