Can I just make up a contract for my services for something else (but hide something immoral withing it)?

No "big company" would sign something like that, as they would have their attorneys review the proposed contract before signing.  If someone was silly enough to sign a contract they didn't read or didn't understand, however, it is possible that they could be incurring obligations they didn't realize were in the contract.  Something as blatant as what  you are suggesting would probably not be enforced, but something like, for example, a provision providing that you would receive no refund if you cancelled an appointment unless  you provided a week's notice of a cancellation, might be.  Always read any contract you sign, and make sure you understand it.