Yes, you can. You would simply make your rename rule to use "simple text substitution" to change the "extension" component of your links.
However, doing so may be a bad idea.
When you use that rename rule, all LinkFixer Advanced does is change the extension. It doesn’t change anything else about the internal structure of your files.
For example, there are structural and functional differences between a DOTX file and a DOCX file. When Word opens a file with a particular extension, it expects to find the corresponding structure inside the file. Word will give you an error message if you manually change the extension on a DOTX file to DOCX and then open the file in Word. If you make that change manually using Windows Explorer, Windows will warn you that the file may become unusable if you change the extension.
Unless you have some other utility that will change the internal structure of these files for you, it probably isn’t a good idea to use LinkFixer Advanced to only change the extensions of files and links.