Can I just add an amendment to a P/S plan plan? Or do I have to purchase an entirely new document with all the 401(k) features?

This isn't your normal snap-on amendment...

I assume you are using a prototype document. I would contact your document provider concerning this. My guess would be that you would have to do a new adoption agreement.

There is no legal reason you cannot amend a PS to a 401(k), but there are practical reasons why it may make more sense to restate the plan using a 401(k) document.
Specifically, adequately providing for the special compliance provisions pre-tax contributions are subject to may be more costly than simply restating.
There is some prior discussion on this website concerning amending a PS to a 401(k) that seems to generally prefer an amendment to a restatement, though. Its really varied, so I'd suggest you perform a search on "amend profit sharing to 401(k)" (or something to that effect) and surf to get a feel for the general consensus.