There is no way to turn Cleave into a basic attack for Combat Challenge. (as of the search of the online compendium conducted today)
There are classes that get powers as basic melee attacks (eg: warlock, sorcerer, druid) that can be obtained (eg: half-elf + versatile master + were-theme) and can be used during Combat Challenge (eg: Savage Rend or Eldritch Strike, with slide 1, used as an immediate interrupt to cancel an enemy's attack).
There are feats that turn specific at-wills into MBAs, including Power of Skill that turns the powers Divine Bolts, Overwhelming Strike, Righteous Brand, and Valiant Strike into MBAs that can be obtained and used during Combat Challenge (Overwhelming Strike lets you shift and slide the target into your square, which is less effective than directly sliding it but might do the trick).
The War Wizard of Cormyr paragon path lets you use any Wizard power with 1 target as a basic attack, which, amusingly, allows you to use Beast Switch as an MBA for Combat Challenge (hit: 1d8 + Intelligence modifier psychic damage. You knock the target prone and can slide it 1 square. The target cannot make opportunity attacks until the end of your next turn) which is brutally powerful when you use it as an immediate interrupt. (Stack in Psychic Lock for extra monster QQs!)
There are 8 Greater Style feats that allow a Fighter to use one of a list of specific Encounter powers as MBAs during Combat Challenge.
There are a multitude of feats and a paragon path (Swordmaster) that allow you to use Cleave as an MBA after a charge and there are a ton of powers that allow you to use them after a charge, but none of this applies to Combat Challenge. Likewise, there are a ton of powers that let you use them on an Opportunity Attack, but also does not apply to Combat Challenge. To my knowledge, nothing specifically turns an at-will into an MBA on granted attacks.
I'm sure this isn't the answer you wanted; nor is it a very useful answer, because it could be invalidated any time by Dragon magazine publishing a feat or power or whatever that lets you use an at-will during Combat Challenge.
Until then, though, this'll have to do! :)