This feature exists in phpMyVisites.
It should be possible in the admin area, to add new partners' URLs, for a campaign. For example, visitors coming from "sourceforge.net/*/piwik/" (broad match supported) would be classified automatically under the campaign "Campaign Sourceforge".
See for reference [source:/trunk/config/global.ini.php]
169 ; variable name to track any campaign, for example CPC campaign 170 ; Example: If a visitor first visits 'index.php?piwik_campaign=Adwords-CPC' then it will be counted as a campaign referer named 'Adwords-CPC' 171 campaign_var_name = piwik_campaign 172 173 ; variable name to track any campaign keyword 174 ; Example: If a visitor first visits 'index.php?piwik_campaign=Adwords-CPC&piwik_kwd=My killer keyword' then it will be counted as a campaign referer named 'Adwords-CPC' with the keyword 'My killer keyword' 175 campaign_keyword_var_name = piwik_kwd
this won't be done for partner but will be an extension of the concept of campaign in piwik.
decreasing priority for now as this might be an issue in the Tracker if the list of campaigns is large - the simple implementation (storing cache in tmp/cache) wouldn't scale so well with hundreds of campaigns.