The 25-year-old will finish his accountancy training on a part-time basis until September before beginning his professional career with the Reivers.
Calum has signed a one-year deal with the club from Boroughmuir, where he won three BT Cups, and also represented Scotland in a club international against Ireland in March.
He said: "I am really excited about joining the Borders, it has all come entirely by surprise.
"Most of the new signings at the Reivers have come through the Scottish Institute of Sport and the National Academy set-ups, so I thought an opportunity like this had passed me by.
"But this sends out a strong message to all the club players out there that there are still chances for them to make the step up.
"It will be quite surreal playing at the same club as Chris, but it will be nice to have someone you know so well to take you through the paces, especially as Chris is such a big character at the club."
But his brother, Scotland and Lions international Chris, 24, has welcomed the new addition.
He said: "It is great for Calum because I know it is something he has always wanted to do.
"He proved his quality this year and was one of the stand-out performers in the club international so he really deserves this opportunity."