"Our focus remains on players who have been through the programmes within our zone and who work hard to grow our high performance culture."
Harrison, a 79-test veteran, has only ever pulled on the Mystics strip in the semi-professional era and was a key figure in helping the side reach the finals series of this year's ANZ Championship.
It is the first time she will team up with Purvis but the two Silver Ferns' team members both offer a vast amount of experience at netball's elite level.
Purvis was previously coached by new Mystics mentor Helene Wilson at schoolgirl level.
The 23-year-old circle defender, who has captained New Zealand at under-21 level, was a training partner with the Mystics in 2011 before heading south to join up with the Steel where she has been for the past six years.
Midcourter Nathan, 23, is also returning to her netballing roots after a season with the Pulse where she made an impact coming off the bench in her rookie year.
Nathan, who can cover all three midcourt positions, grew up in Auckland playing netball across three centres including Waitakere, North Harbour and Auckland. She was a member of the Northern under-23 side which won the 2015 Netball NZ National Champs.