Swansea-born Williams played for North Harbour in the Farah Palmer Cup in 2020 and 2021 and recently played for New Zealand Defence in the Defence Rugby World Cup. She trained with Wales during their pre-season camp in July.
Williams got the call when flanker Alisha Butchers was injured.
Wales coach Ioan Cunningham said: "Kate has been on our radar for a while. She's Welsh, born in Swansea and has played a high level of rugby in New Zealand.
"We are delighted to have her in our squad and excited to see what she can do in the future.
"When she trained with us in July, I made her aware that she could be involved if anything happened while we were over here, and I'm pleased with the way she's got stuck into training and settled into our environment."
Williams said she was honoured and excited to be training with Wales.
"My whole family is Welsh, and I've always felt a strong connection to Wales despite growing up here [in New Zealand]. We've always supported Wales as a family, my parents have made sure my brother and I feel a part of Wales."
She had a chance to train with Wales in July through a grapevine situation, so she booked her tickets.
"It was probably the best thing I've done. I thoroughly enjoyed that experience, and it means I already know most of the girls a little bit.
"It's a different vibe now that we're in the middle of the World Cup compared to pre-season, but that's what you'd expect, everyone is switched on."
Williams said she had played rugby since the age of four with the North Shore Rugby Club and for Glenfield College and, since 2017, for North Harbour.
"My rugby highlight until now was representing the Blues women and, then recently, the NZ Defence team experience was brilliant too.
"Training with Wales in July made me focus on my rugby future, it's what I want to do. I've arranged to take a year's unpaid leave from January, so I want to have a real go at playing rugby semi-professionally, or professionally in the UK, and see where that takes me," she said.
"I feel my point of difference is speed over the ball, and taking some defensive steals. That's what I love about being a back rower, and it would be amazing to have the chance to show what I can do on the field."