Yes, that's definitely doable. However, you'll need to do that on the QuickBooks Online side.
If you change the master form style in QuickBooks Online, this will be applied to all Purchase Orders you raise and send to QuickBooks Online via ApprovalMax. Technically speaking, the Purchase Order itself is configured in QuickBooks Online and ApprovalMax just sends the respective values to QuickBooks Online once the request has been fully approved.