The above posters are right and wrong. No, Twi'lek can't wear hoods up. However, as it is at the moment, Twi'lek hoods are not down, they're completely missing from any robe chest pieces. I'm not sure if Bioware plan to add the hoods back onto robes for Twi'leks even if they're just dangling there unused but it looks pretty stupid with them just completely missing.