passing database package results to BI Publisher data model

I am new to BI Publisher server, I have been using  BI Publisher servlet that was embeded in Oracle Transportation Management. This has been discontuniued in the latet release.

When using the embeded servlet I could pass a variable to a database package using a report trigger and then assigning the result to a variable for use the query.

For example, I have a function that takes a string and parses it apart to return a substring.  I can then use the substring in my query.

How do you do this when using the BI Publisher server?

I see I can use the trigger before the report runs but how do I access the results of my package function?