The short answer is yes, a minister may designate their entire pay as a housing allowance provided that the amount meets guidelines. You can find specific information on our Housing Allowance Page, including a worksheet to help ministers determine an appropriate amount to claim.
Do keep in mind that a housing allowance is only available to ministers who are ordained or fellowshipped. Preliminary fellowship does meet this requirement. More guidance on the particulars of compensation for ministers can be found on our Ministerial Compensation 101 page.