In that case, I am afraid, you need to do it manually. For example, you may load the XLS file first, now load the second tab delimited file, export data to fill a data table or array from your tab delimited worksheet. Now, you can import data from data table/array to your XLS file' worksheet at your desired location.
See the topics on how to export and import data from/to worksheet:
An agricultural way can be, you may put value cell by cell using Cell.PutValue() method in the XLS file from your tab delimited worksheet obtaining in a loop using Cell.StringValue() etc.