You can't do that anyway. OOI says you can "designate one creature you have cast a wizard spell upon..." or "...extend the durationI imagine that they want to limit the possibility of unexpected combinations of powers and properties. Being able to use Orb of Imposition with a Warlock power is something that might need to be considered when making the power if they could be used together.
of an effect created by a wizard at-will spell..."
Even if you can add your orbs +3 to a warlock power, you can't use OOI to bump its save.
That said, to the OP:
No, you can't mix-n-match implements like that, by the rules as written. The Wizard "Implement" line says, "A wizard wielding a magic orb, staff, or wand can add its enhancement bonus to ... wizard powers ... that have the implement keyword."
The warlock's text reads similarly: "A warlock wielding a magic rod or wand can add its enhancement bonus to ... warlock powers ... that have the implement keyword."
Similarly, Pact Initiate says "you can use a rod, a wand, or a pact blade as an implement when using a warlock power" and Arcane Initiate says "you can use an orb, a staff, or a wand as an implement when using a wizard power".
In neither case can one class use another class's implement on its own powers. Wizard Implement powers require wizard implements; Warlock Implement powers require warlock implements. A multiclass of the two would be well advised to go with a wand.
Right. I agree that this is the reason for the ruling, and that you shouldn't houserule around it.Or future rod properties would need to be evaluated on the basis that they might become useable with a fireball or other Wizard powers. But since the rules say that Warlock implements can only be used with Warlock powers and Wizard implements with Wizard powers, it's one less place to get tripped up.
For example, consider the Rod of First Blood. It's good for a warlock to deal bonus damage to undamaged creatures. But a wizard who can open combat with a fireball can multiply that damage a huge number of times!*
And like Tale says -- it's just adding another layer of possible exploits to the game.
* Note that a 'lock with a Dilletante or multiclass feats still couldn't use that method to multiply his damage that, because it says "When you hit a creature ... with an attack using this rod", and due to the rules I quoted above, you can't use a rod to enhance a fireball, no matter what.