A family of Microsoft word processing software products for creating web, email, and print documents.
Assuming you're not using the built-in heading styles for the headings in the Preface, there should be no problem. Just define a new multilevel list linked to the styles you're using. You can modify these styles to have outline levels of 1, 2, 3 so that the TOC entries will have the same formatting as the comparable chapter headings.
If you do want/need to use heading styles (because you need to include the paragraph number as a "chapter number" in page or caption numbers), you can use some of the lower-level styles (say, Heading 7-9); they won't include numbering from the upper levels or restart after higher levels. This would be the same principle as that explained for numbering appendixes in http://www.ShaunaKelly.com/word/numbering/NumberingAppendixes.html.