One way would be to try and use R to import the Excel files and also write the results directly into SQLite, ARFF or Parquet (or CSV). So they would not need to be processed in KNIME (but you could always load them back from the mentioned formats).
If the Excel files become more complex you could also use R to help you with that (sheet names) etc.
If you have problems with column formats you could force them all to string and then decide the right format
If you have to deal with password protected files you could do something with Python
If you want to do more advanced things with KNIME and Excel (with the help of R and Python)
(search for excel, xlsx in filename)