Fenwick and Leaver will provide back-up for shooters Jhaniele Fowler-Reid and Te Paea Selby-Rickit, while Watson will compete for game time with defensive duo Storm Purvis and Phoenix Karaka.
Watson (24), who has spent the past two years at the Tactix, is determined to recapture the sort of form which earned her a Silver Ferns trial last year.
''I can't wait to join the Steel. It's a new environment, a new opportunity and a new coach to work with, which is really exciting,'' Watson said.
''They're quite a young team and they're not afraid to make those changes to whoever deserves to be out on court.
''I definitely want to improve and have a great pre-season to really push for court time.''
Fenwick (21) secured a contract with the Steel in 2011 before returning home to Canterbury to play for the Tactix.
''I've learned a lot of things since then, particularly around the hard work and commitment it takes to be playing at this level,'' Fenwick said.
''I haven't been getting as much court time as I would have liked this season, so I wanted to put my name out there and this opportunity came up ... I felt it was time for a change and this is a chance to really reignite my game.
''I'm not expecting anything [at the Steel] but I am going to be trying my best.
"In a team environment, you have to have that competition for court time with the balance of supporting the ones who get the nod.''
Fenwick teamed up with Leaver (20) - daughter of former Silver Fern Leonie Leaver - in the New Zealand under-21 team which won the title at the World Youth Championship in Glasgow last year.
That team was led by Steel coach Janine Southby and Fenwick is excited to be working with Southby again.
''Janine has showed she will mix things up when required and I think it's really refreshing to find a coach willing to do that,'' Fenwick said.
The Steel has 11 players confirmed for 2015 with Fenwick, Leaver and Watson joining Karaka, Purvis, Fowler-Reid, Selby-Rickit, Wendy Frew, Shannon Francois, Gina Crampton and Stacey Peeters.
• Silver Fern and Northern Mystics defender Anna Harrison will miss the rest of the 2014 international season as she is expecting her second child, The New Zealand Herald reports.
The 31-year-old recently returned to New Zealand after competing in the 2014 Glasgow Commonwealth Games and will bow out of the upcoming Constellation Cup series against Australia and the two-match series against England for the Taini Jamison trophy.
