Sajid, I have found two solutions to this pattern.
1. Write a .net object that will accumulate nodes and when done return a message with the unbounded records.
2. Create an accumulator map. Input is a composite message of 'current' unbounded collection and next element, output is the 'new' combined unbounded collection. Loop until done. Looping functoids can help do this.
Since you are already familiar with xpath, the first solution may be best for you
luck
Doug