First of all, why would they arrest him? What was the crime exactly? Enquiring minds want to know. Beware of the scare tactics employed by the haters.

1. He ostensibly did not rape her, even if there is an alleged dress. She was a pro, and that's not illegal. also, this happened BEFORE he became president, so the allegations smell kinda political - not criminal
2. Paying off an extortionist is not illegal, while extorting IS, so which criminal are they going after? The victim? Very strange
3. He denies paying her - so is that illegal? Nope. Not even if he was guilty, would it be illegal to deny everything. It would just mean that it would have to proven in court that he was a liar BEFORE convicting him of having relations with a pro - which won't happen. So what would be the point?
4. He has a letter from her saying she has had no relations with him - did the lawyer fellow pay to have that statement? Still not illegal - more like a non-disclosure - which is done all the time
5. It would be quasi-illegal to not show up for the hearing, but he is transparently going there - so there is no reason to arrest someone who willingly shows up

Second of all, they cannot force you to give DNA unless there is a crime and they need proof, such as murder or breaking and entering or something - NOT proof of soliciting and having sex from a pro, which IS NOT ILLEGAL. I'd say he will have several lawyers present, and they will stop it if necessary. Then again, if he does provide a DNA sample, what of it?