Galarza played his first match for the club last weekend, coming on as a replacement against Oyonnax in France, whilst Creevy could feature this Saturday against Sale Sharks in the Amlin Challenge Cup.
Back-row Senatore will be eligible to play for his new club when Bath Rugby visit Sixways in the Aviva Premiership on Friday 1 November.
Second-row Galarza said: "I am very happy to be here. It is a great club and Worcester is a really nice city. I came here with the Pumas in 2011 so I knew the place before I joined.
"I would like to play as many minutes as I can and I think we can bring a lot to the team. I want us to get as high as we can in the league.
"I really enjoyed my first match in France last weekend but now I'm looking forward to playing my first game at Sixways in front of the fans."
Hooker Creevy added: "I'm delighted to finally be here as I've been waiting for months to join up with the team. The club is amazing and the players and the staff are great.
"I am excited to get out there and play my first match for Warriors and I hope we can add something and help the team to win games."