I don't know how to do it with .htpasswd but you can can create an usergroup called "Paid Users". Place users that pay for access to the Members Area into the secondary usergroup "Paid Users".
At the very top of the template "xwp_download" add
HTML Code:
<if condition="is_member_of($bbuserinfo, X)">
Replace "X" with the ID of the usergroup "Paid Users".
At the very end put