The role of trustee is to be responsible for the investments of the plan. It is not recommended to give that role to somebody who you don’t want to have that responsibility.
If you want to hire your accountant to keep books, nothing further is required inside of your plan. Your role as administrator and trustee means that you are responsible for keeping the books. If you want to hire a CPA or an accountant to do some of the things in terms of record keeping, you can do that without adding them to the plan.