If I understand correctly, what you want to have happen is to be able to footnote sentences, and then, if a new footnote is added in the middle, not have the numbering change. Since that's exactly opposite of how people normally use word processors I would be surprised if Word (or any other word processor) were capable of this sort of numbering.
That said, the numbering you're asking for is what comes naturally as you write a document - so why not just type in the numbers yourself (you can use a small size/superscript to make them look like foonotes if you wish), and manually type the reference section?
Alternatively, why not manage your various "shalls" as a list in something like OmniOutliner? Hell, since it's software requirements you're discussing, there might well be a specialized piece of software to manage these things for you.