SugarBPM: add relation between two records

Hi, I can't solve this need: I would like to link the source of the lead (dropdown in opportunity module) to specific campaigns.

ES: lead source = website, campaign = "contact us" form.

So, I wanted to pilot the action with SugarBPM by making sure that when the source SugarBPM is saved it automatically enhances the campaign; I'm currently locked out because I can't find a way to value a related record (so add the relationship between opportunity and campaign).

Is there any way to do this?

Thanks,

Mirco