Convert APK to RAR

Converting Android Package Files directly to WinRAR Compressed Archives (.apk to .rar conversion) is not possible, and honestly wouldn't even make too much sense.

You might, theoretically, extract data from your .apk file (which is merely a renamed ZIP file), and recompress the data to a .rar archive. In that way, however, your compressed set of data will no longer be useable for installation purposes on any Android devices.

Simply put, it is impossible to convert APK to RAR directly.
Thus, there is no so-called apk to rar converter or a free online .apk to .rar converter to be found.