Paypal express - adding an "agent code" field (doesn't need to modify the price)

I've been reading up about paypal express and I'm trying to work out if I could add an extra field to collect the agent code. It's just something simple, it's just a code like "AGENT-X" that could sit in a field like the comments, or hopefully it's own field called Agent Code that is on the paypal express checkout form.

Is this possible? There is no real mention of it on the paypal introduction to express here

I've seen this quest开发者_如何学运维ion, I was hoping that I wouldn't have to add an extra screen to my checkout process just to collect this field, and that rather it could be collected on the paypal checkout screen

This is not possible with PayPal Website Payments Standard and/or Express Checkout unfortunately.
Is there no way to dynamically populate / retrieve the agent code? You can then pass it on in the CUSTOM parameter in your SetExpressCheckout API call and have it returned in GetExpressCheckoutDetails when the buyer is back on your site.