> Are you going to pay extra those two engineers of your company for doing something that is clearly outside what they were hired for at beginning or outside their competences?

Why is it outside (and not just outside, but clearly outside) of what they were hired for or what their competencies are? Engineers work on the product. Engineers review each other's code. Engineers are a stakeholder in this whole process — after all, the candidate may become their future colleague, and engineers are best positioned to know what they want to see in a prospective colleague. Engineers can appreciate whether their peers have the desired skills.

Why does this activity require a higher pay than developing a new feature with your teammates?