Unfortunately you cannot. While this person's experience may prove very helpful in the preliminary stages, there are a number of reasons why they should not get involved professionally with your project.
Architects can only provide professional advice if they hold professional indemnity insurance, which covers the cost of putting things right should their advice be flawed and cause damage. Retired architects will not carry indemnity insurance and there are still potential conflicts of interest if they provide advice on a professional basis (see the following question).
No. Managing trustees may ask for reimbursement of reasonable expenses but must never carry out work for their church on a paid basis because there is a clear conflict of interest, while this is also contrary to charity law.
If you appointed either of these people then it could be very embarrassing for all concerned if problems occurred due to bad advice and you (the other managing trustees) were forced to take legal action. It is very important to remember that verbal and even unpaid advice carries the same liability as it would if it were paid for. We therefore recommend that you do not consult either person and instead seek independent professional advice from an external architect.