Otago's McCullum has signed with Glamorgan and McClenaghan with Lancashire for the English domestic Twenty20 competition which runs between June 26 and August 17.
McCullum's man-of-the-match performance in New Zealand's one-wicket win over Sri Lanka in Cardiff has impressed the powers-that-be at Wales' sole county side.
He eked out 32 runs under pressure as New Zealand struggled to a target of 139 and took 2 for 23 from eight overs of off-spin.
The club has previously had New Zealanders on its books. McCullum's brother Brendon and team-mate James Franklin played there in 2006.
Nathan debuted for New Zealand at the 2007 World Twenty20 tournament in South Africa. He has since become a regular in national limited overs sides with 45 Twenty20 and 51 ODI caps, as well as appearing in Indian Premier League and Australian Big Bash squads.
McClenaghan has been a limited overs revelation since debuting in Twenty20s late last year against South Africa.
He's been issued with a licence to bowl aggressively and has responded with four four-wicket bags in nine ODIs, including his last two against Australia and Sri Lanka. He was the highest wicket-taker in the three-match ODI series against England with eight at 17.25.
The only time he has seemed flustered this tour was in a 10-ball death over at Nottingham. Coach Mike Hesson said McClenaghan had shown maturity beyond his 27 years, despite minimal international experience.
"He's not afraid to show variations regardless of whether he gets it wrong. The fact he is prepared to do that is great. He gets wickets with the new ball which is critical for us, like Shane Watson the other day against Australia," Hesson said.
"He's confident in his own ability, has been around the first-class scene a while, and knows his game. There is an element of Shane Bond there, although Shane probably had more pace. Mitch is of the same mindset as an aggressive wicket-taker."
