Yes, you can. That should be no problem. You’ve already got a decent level in English, so go for it with Mandarin! Continue to “work” your English by enjoying TV series, reading articles online, and so on. I suppose you could study Mandarin from English (rather than Portuguese) to get a good review of the basics. (Once you’ve got the basics under control, try to use more immersive methods.) The main issue is that Mandarin is going to take a lot of time for you to learn — probably four times as much time as it took you to reach the same level in English. Be prepared to enjoy a long journey.