Given that very few programs support stylesheets, MS Word has always had good tools available for turning any document into a stylesheet enabled document, especially if that document has been designed to make that easy. The most important thing you need to do is make sure that all ranges of text that should be associated with one style are clearly different from other ranges of text. Since all we have is the formatting to go by, you can’t have level 3 and 4 headings formatted precisely the same. In other words, it doesn’t really matter what things look like coming out of Scrivener. We’re going to be applying everything to a stylesheet that is going to look different, anyway.
Once the RTF file is loaded in Word, simply use its tools to select all text with a similar style. That is different in every version of the program, so I’ll leave finding it up to you, but in MS Word 2010, you right-click within the text you wish to apply to a style, and use the “Style” sub-menu to select all text with a similar style. If for example you had done this to a chapter header, you could then click the “Heading 1” style to assign all of the selected text to that style. Given that the average document only has around a half a dozen different styles, if that, you can get a document from Scrivener to Word stylesheets in no time.