MK Dons midfielder George Baldock, 18, has signed on loan for one month after impressing against the Cobblers in a pair of practice matches recently.
Johnson meanwhile has handed former Queens Park Rangers prospect Chris Arthur a route back into league football.
The versatile 21-year-old left-sider had been playing recently with Havant & Waterlooville.
Johnson told the club website: "I believe we can get his career back on track."