I believe that you want to dynamically deterimine the client sequence file after the PreUUT sequence in the process model has obtained the serial number you have entered.
Another customer has asked a similar questions which has the answer you are looking for. To find it in the Developer Exchange user the Search option to search the TestStand category for "client". The questions was "How to load the sequence file from the process model?" and I believe that aperez provides the best answer.