There are two circumstances in which you may do that.
If the site is in more than one language (not babelfish or other machine translations, competently translated by a multilingual human) then you may suggest it for inclusion in the most appropriate category for each language.
If a site is of both local and topical interest, you may suggest it for inclusion in one local and one topical category. For example, a site about a widget manufacturer in Los Angeles may be listed in both a manufacturers category and a Los Angeles category.
The above two criteria are combinable. If, as seems likely, that Los Angeles widget factory has pages in Spanish and English, there are four places you may suggest it.
Other than those two circumstances, no. That's not saying we won't list a site in multiple categories, we often do. But, except for local/topical and multiple languages, we much prefer that the decision to do so be initiated by an editor familiar with the fine nuances of when such multiple listing is appropriate.
