No, if someone is acting as an 'agent' of law enforcement, the evidence would be excluded via the exclusionary rule just the same as if the evidence was collected directly by the police themselves.

They can't employ them to do illegal things. They'd have to have some sort of plausible deniability. If they did, the private contractor would be acting as an agent of the government. If they circumvented this by creating some cover story and payments from some other sources, then they might get it into courts, but if it came to light it'd be thrown out (and the agents/agency involved would likely end up in some degree of trouble).