Populating Target Field value runtime

Hi,

can someone suggest me on how can I populate the target field- Details segment in a sequence manner?

e.g. : <Details>

          <a>001</a>

          --

          --

          </Details>

          <Details>

          <a>002</a>

          --

          --

          </Details>

          <Details>

          <a>003</a>

          --

          --

          </Details>

          <Details>

          <a>004</a>

          --

          --

          </Details>

     I have to populate the Details segments dynamically at run time based on the values that I am getting in Source structure.

I believe this can be done by UDF, please suggest.