Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.
I don't think anyone from any office can arrest a sitting president.
No one is considering arresting Obama. In fact it is likely impossible and most probably inadvisable. In fact it would be ludicrous to think of arresting Obama because of the separation of powers doctrine.
What US Marshals can do is to serve arrest warrants or summons on federal contractors that are involved in making the amnesty ID Cards which lead to SSNs. Federal contractors are fair game and cannot escape jurisdiction of a federal court order.