The short answer to your question that you cannot use a text file as either a Source or Destination within an LEI Replication Acitivity. However, you could use the text file as the Source in a Direct Transfer into a Notes database, which you then use as the Source for your Replication Activity. You should be able to setup a Connection document using the Text Connector which will point to the *.csv file. Your Direct Transfer Activity should "Overwrite Existing Data" (an option within the DT OptionsTarget Data section) to ensure you are always working with a fresh set of data from you .csv feed.
This was first published in June 2002