Kevin Sinfield has been appointed permanently as Borthwick's number two, specialising in defence. Evans' temporary addition further swells the changes following Eddie Jones' sacking in December.
“I am delighted that Nick is joining our team ahead of the Six Nations," said Borthwick. “Nick brings with him a wealth of different experiences as a player and coach, which will be incredibly valuable to the growth of our team.
“I know that Nick understands the pride and responsibility that comes with representing your country. We aim to build an England team that our nation can be proud of, and I know that our supporters will be very excited by what Nick can bring to our side.
“We’d particularly like to thank Harlequins for their support in allowing Nick to work with us during this period. It’s a great example of club and country working together.”
Martin Gleeson handled the attack before Jones' sacking, and has remained in place after Borthwick's appointment. The RFU has not yet clarified Gleeson's position in light of Evans' appointment, but the former Rugby league international will be expected to leave the England set-up.
Evans won 16 New Zealand caps before making 208 Harlequins appearances between 2008 and 2017. The 42-year-old has coached at Quins since his retirement, and will continue his role at The Stoop while on England secondment. Harlequins say Evans will return to the club full-time after the Six Nations.
“This is an amazing opportunity for Nick, where he will learn and grow so much, which can only be good for the club," said Harlequins rugby performance director Billy Millard.
"I am also confident he will provide Steve and the playing group with excellent support and direction through the 2023 Six Nations period.”
Jones was sacked in December after seven years at the helm. England's 2022 represented their worst set of calendar-year results since 2008, with a return of just five wins in 12 Tests.
Matt Proudfoot, Brett Hodgson and Danny Kerry all left the RFU on Monday. Scrum coach Proudfoot had been with England since 2019, while Hodgson left the defence role he was only due to start under Jones in this year's Six Nations.