A key part of the Quality Assurance Agency's (QAA) role is to review whether universities (as institutions or programs) maintain their academic standards and quality.
I also think the same way what Prof. Kamal pointed. If the external reviewer committee is formed by experts in the specific areas they are evaluating there should not be any issues. If experts from accounting background reviews chemistry there could be a potential problem.
As Huajiang Ouyang stated, it is possible if it is necessary as a member of QAA team. Of course, it depends on what QAA team reviews in an institution. On the other hand, an institution has many functions and roles. Hence, we need reviewers depending on the content of the tasks and defined jobs. For example, If the institution deals with any monetary issues, why don't we have an accountant in the team? We need an expert accountant even in a small scale research nowadays.. If the chemistry department has contacts with industry and industrial productions, it will be a very good idea.
If we are talking about general accreditation yes we can say that people with any background still can evaluate university but not for all aspects.
If we are talking about special accreditation for a program. Thus the person who in charge should have a compatible background with the area of proposed program.
Unfortunately in developing countries people whom in charge to evaluate the university either general or specific they are not qualified or don't have enough experience to deal with it.
We have to differentiate between institution accreditation and program accreditation
For institution accreditation external reviewers should be experts of the process and of the national standards while for program specification external reviewers should be expert in both process and contents.