SITX and SIT compressed archives are from various editions of StuffIt, so you may as well be able to repack one to another with any of the StuffIt programs. There does not seem to be great difference between .sitx and .sit files, but you can freely convert between them as you wish.
Convert StuffIt archive to StuffIt archive format.
We found 4 software records in the database related to .sitx to .sit file format conversion.
Microsoft Windows software - convert sitx to sit on Windows
A commercial compression utility
Convert from sitx file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned sitx to sit. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Popular file archiving utility
Apple macOS / Mac OS X software - convert sitx to sit on OS X
A backup and archive utility
Mac version of the popular utility