Do you mean you want an extra text field in the checkout page for customer to fill in the "Purchase Order" number? If yes, this is achievable using custom app that has the text field checkout ui extension, but this require your store to be on Shopify Plus plan, is your store on Shopify Plus plan?
Spent too much support time dealing with order cancellation request from customer? Wouldn't it be good if customer can cancel order on their own without bugging your support? Try out Cancellable app! https://apps.shopify.com/cancellable . I also write articles about store customization that can improve your customer shopping experience here : Yagi Software Blog