Ferreira comes to Roar from Melbourne Victory - where he made 52 appearances - with Petratos returning to Australia after a six-month stint at Malaysian Super League club Kelantan.
'The addition of Diogo and Dimitri brings quality and versatility to our squad," Roar coach Mike Mulvey said.
'I’ve been impressed with the determination they’ve both shown since they arrived. They are good additions to the squad and have a desire to be successful.
'Diogo and Dimitri provide an element of youth but bring significant A-League experience to our playing group as well."
Ferriera, who spent time in Portugal with FC Porto’s youth academy as a 16 year-old, joins Brisbane as a utility player. Mulvey is said to be confident in his ability to play in a range of positions across the defence and midfield.
'It’s exciting to be joining one of the most successful A-League teams in recent years," Ferreira said.
'I’ve been impressed by Mike Mulvey and what he’s trying to implement here and I really think I can offer something different to the squad."
Petratos, who played 30 times for Sydney FC before heading to Asia in December 2012 to gain AFC Cup experience with the Malaysian champion, will provide similar versatility in the attacking third.
'I’m excited to be back playing football in Australia and to be here in Brisbane particularly," Petratos said.
'I’ve enjoyed watching the way the Roar play over the past couple of years and now I’m excited to be a part of it."
The player turnover at the club continued with Steven Lustica returning to his Croatia club, Hadjuk Split, after his loan deal ended.