*A footnote is a text inserted outside the main text for purposes of explanation, comment, etc., OR the first subnote should be numbered '1.1', the first sub(sub)note within it '1.1.1', etc. Carrying things a bit farther, to add a footnote to a footnote that is already within a footnote, one might use the same set of symbols, but in reverse, or perhaps offset by starting the second-level subnote with the symbol corresponding to the current level, thus allowing for up to five levels.¹ This approach of course doesn't sequentially number subnote along with the first-level notes, but it could be argued that if subnotes are numbered the numbering should start over in each note containing a subnote.
HOW TO ADD FOOTNOTE IN WORD ACADEMY MANUAL
If I'm not mistaken, the Chicago Manual suggests using non-numeric symbols like '*' or '†' or '‡' for footnotes within quotations or within footnotes, and I believe gives a hierarchy of such symbols that allows for up to four or five footnotes within a quote or footnote. However, this suggestion seems a little too convoluted.
HOW TO ADD FOOTNOTE IN WORD ACADEMY UPDATE
You'll also want to make sure that you update fields within your document just before printing. If you do, then your marker for footnote 3 will actually look like 34 (because the hidden marker for footnote 4 is right next to it). The only thing you need to ensure is that you don't print hidden text with your document. You've now created a footnote within a footnote, and the numbering will always be correct.
There is a way around even this drawback, however.
Your fudged footnote isn't recognized by Word, so the next time you enter a footnote, its numbering continues from where Word thinks it should continue, not from where you know it should. The only drawback to this approach is that it plays havoc with automatic footnote numbering. The only thing you can do is to "fudge" the footnotes-format your own footnote mark, press Enter at the end of an existing footnote, create a corresponding footnote reference at the start of the new line, and then enter the text for the forced footnote. If you select that footnote mark, copy it, and paste it into the footnote at the place you want, you still don't get the opportunity to enter any new footnote text. If you try to place a new footnote while the insertion point is located within an existing footnote, all that Word does is duplicate the footnote mark at the beginning of the existing footnote. It doesn't provide a way to add footnotes within footnotes, however. Word does a great job of creating footnotes, using either your own footnote marks or automatic marks. (For those with a linguistic or literary bent, you can find out about footnotes within footnotes by referring to a style guide, such as Kate Turabian's book, Manual for Writers, or the Chicago Manual of Style.) In some types of documents you may actually need footnotes within footnotes, such that a footnote can be footnoted, with the "sub footnote" appearing in the regular flow of the main footnotes. Footnotes are a common feature in many types of documents.