"For me I believe it is a good career and personal move - I want to play international rugby and it would be a fantastic honour to play for Ireland as well if it came about. Before any of that though I am really excited to play for Leinster. I really do believe it is the right move for me," said Sykes.
"Of course it was not an easy decision to make, I don't for one second want anyone to think I am unhappy here or unhappy at the Sharks; I love the Sharks! I have played over 130 games for the Union and loved every minute of it. However I believe that it will be very good for my rugby to make the move and I am looking forward to the experience of playing in European conditions week-in and week-out, learning about their way of playing - which I think will suit me.
"Leinster is a leading European club, regularly challenging for top honours, and I can't wait to be involved. I think it will be great for me and my wife to get the experience of living there as well."
Hagan, 23, a former Leinster and Ireland youth tight-head makes the move back to Dublin after two years at Connacht where he made 46 appearances, including 13 games in the Amlin Challenge Cup.
"I am delighted to be heading back home to Leinster and I am looking forward to the new challenges that I will face. Everyone in Connacht (particularly Eric Elwood and Dan McFarland) has been very good to me over the last few years and I'm looking forward to pushing on from my experience in Galway to make an impact when I return to Leinster in the summer," said Hagan.
Berquist joins on a one-year deal with a further option to extend the contract and played against his new team-mate, Sykes when the Crusaders hosted the Sharks at Twickenham on Saturday.
Waipukurau born Berquist, 27, joined the Crusaders in 2010 having also played for Hawke's Bay, Otago and Auckland.
"I have always been keen to test myself in Europe and the Magners League and Heineken Cup competitions will certainly provide a real challenge. I am delighted to join an ambitious club who are on the up. Leinster have a strong reputation in New Zealand and I am excited by the opportunity to test myself in a new environment," said Berquist.