Q: Is there a way to install a Portuguese dictionary in iBooks?

I don't think that Portuguese is an option yet - as well as the ones that you list I also see (by selecting a word in an ibook, then 'define' on the popup, and then 'manage') Korean, Korean-to-English, Italian, Dutch, Simplified Chinese-to-English, Japanese and Japanese-to-English, as being downloadable but not Portuguese. Assuming that you also don't get an option to download a Portuguese dictionary via that then you could try leaving feedback for Apple : http://www.apple.com/feedback/ibooks_ios.html