Allderman moved to Pretoria to join the Blue Bulls academy and represented them at youth level from 2016 to 2018. In 2018, he made his first class debut, coming on as a replacement in the Blue Bulls XV's Rugby Challenge match against the Welwitschias in a 47–31 victory.[1] A second appearance followed in their quarter-final defeat to the Pumas in Mbombela.[2]
Allderman returned to the Eastern Cape in 2019, to join Pro14 franchise the Southern Kings.[3] He was named on the bench for their season-opener against Cardiff Blues, but failed to make an appearance.[4] However, he did play off the bench in their next two matches against Munster[5] and Ulster[6] before starting his first match in a 30–36 defeat to Benetton in Treviso.[7]