You'll find all the information at
www.soldados.com .
I've been checking, though, and have found the recruiting requisites:
You must be citizen of one of these countries: Argentina, Bolivia, Costa Rica, Colombia, Cuba, Chile, Ecuador, El Salvador, Guinea Ecuatorial, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, Dominican Republic, Uruguay or Venezuela.
You must have the termporal or permanent residence permission or have the right recognized by law to access the Armed Forces for foreigners.
You must be an adult (more than 18 years-old) and have less than 28 years-old at the time of joining the Armed Forces.
Your height must be between 1,55 m and 2,03 m.
I'm not an expert, you should contact the Army to be sure you can't join as a portuguese. In any case, if you change your nationality (being portuguese is pretty simple) you won't have any problem.
Fernando