Mulder trained with Cardiff when they played in South Africa last season.
He said: “There is a lot of strong competition at scrum-half, which is great for everyone as it will push us all to improve.”
Sherratt added: “He impressed us when he helped us out in Durban.
“From initially looking at his footage, I could see straight away that he was a Cardiff-type player.
“He has an attacking mind-set and is good in unstructured play, while he is similar to [Cardiff’s South African fly-half] Tinus de Beer defensively.”