Dozen of different .asc file types exist, so it's hard to give precise answer about any potential xls to asc conversion, except for the obvious fact, it is searched as export of Excel data files.
In case it's just meant as export of (ascii) text from Excel spreadsheet, this can be done with Excel and text editor and simply with the copy and paste method.
For special, unique, .asc file types you have to use appropriate software solution depending on your needs. But you can always export your data from Excel to another format, like CSV and use that instead the spreadsheet.
Updated: January 5, 2024