Rule 8.9.B.2: Any player on the line up card (including substitutes) may be a courtesy runner. Teams are allowed one courtesy runner per gender, per inning. If the courtesy runner is on base when their time at bat comes up, they are called out on the base and allowed to bat. No substitutions are allowed to prevent the out in this instance.
There is no rulebook online, you have to pay for one from OKC or pay to download the app.