If you are referring to the "Junk mail" filter that is offered by Dreamhost (which is based on SpamAssassin), then you can only turn it on and off for the entire domain. So if you are using that, then it's already turned on for all your other mail accounts on the same domain.
If you're using a copy of SpamAssassin that you installed yourself, then I don't know. But I would recommend to use the "junk mail" filter instead, because it works extremely well and is already installed for you.
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