If you really, really can’t get your anal-retentive ASP to allow adding custom tags, worry not. All’s not lost.
First, identify two tags whose values will not change – One each for Sender and Receiver ID.
Then, for “My Enterprise” and Trading Partner’s profile, add the values as “External IDs”. Select an “External ID Type” that you’re not using i.e. “User Defined 3”. Personally, I prefer to create custom “External ID Type” like “ASP Assigned” so that it’s clear what the value is for.
Lastly, for the DocType’s attribute, extract sender/receiver ID, and set Transformation -> Built-In -> and select the “External ID Type” that you’ve chosen.
All this, of course, assumes that your ASP’s format is not used for any other trading partners (which is normally the case anyway, as any standard EDI/B2B formats worth its salt will have sender/receiver clearly identified).
Hope this makes sense to you.