Just a quick update to this.
After you add the custom fields using General Settings > User Profile Options, you may want it to appear on your order pages (admin side) so that you can see where customers have come from when you come to process the orders.
Open: skin1/main/history_order.tpl
Find:
{if $usertype eq "A"}
{$lng.lbl_status}: {include file="main/order_status.tpl" status=$order.status mode="select" name="status"}
<P>
Change it to:
{if $usertype eq "A"}
{$lng.lbl_status}: {include file="main/order_status.tpl" status=$order.status mode="select" name="status"}
<br><br>
{foreach from=$customer.additional_fields item=v}
{if $v.section eq 'A'}
{if $is_header ne 'Y'}
{/if}
{$v.value}
{/if}
{/foreach}
<P>
That's it!
Now, when a new customer registers and places an order you will be able to see where that customer has come from when you view their order in the admin section. (It appears underneath the dropdown menu called 'Status', and above the 'Tracking Number' text box)
This was tested on 4.0.16, but should be fine with other versions too.