IANAL (but spend a lot more time than I would prefer writing and reviewing contracts)...but I don't think your proposal would work in the US. I believe your additional 'document' would have to be structured as an addendum, which in itself is a sort of contract and technically free-standing from the main/original document. But in this case the original contract wouldn't be valid, because it wouldn't have any specific terms relevant to an agreement between parties, and AFAIK you can't render an invalid or unenforceable contract valid through addendum.
What you're proposing also bucks 100+ years of legal writing tradition (both in US and Europe), which itself is very poorly received by attorneys and judges.
What you're proposing also bucks 100+ years of legal writing tradition (both in US and Europe), which itself is very poorly received by attorneys and judges.