He would have to append the desired custom data (subID) as value of "chan" URL parameter, that way it will show up in the list of clicks (and sales if the sale is tracked based on cookie created by clicking this banner) in the "channel" column.
So the affiliate would have to add into the banner code's <a href="" > part of code kind of 'id', e.g.:
NOTE: URL_TO_PostAffiliatePro is automatically replaced with the real URL address leading to Post Affiliate Pro (Network) when the affiliate copies the code from the affiliate panel.
var a1 = document.getElementById('myBanner1').href;
document.getElementById('myBanner1').href = a1+"&chan=DynamicSubID";
That DynamicSubID is supposed to be replaced with the affiliates desired data (subID).
It is up to the affiliate what way he rigs up his custom data - whether he specifies explicitly or reads it via some custom code from somewhere.