I downloaded the free version of rar.exe. In the User's Manual (Rar.txt) available in the install folder, the second paragraph of the Introduction indicates that "RAR supports only RAR format archives, which have .rar file name extension by default. ZIP and other formats are not supported."
Sure enough, I cannot decompress a 7z archive with rar.exe - even from the command window (independent of JMP).
However, WinRAR supports command line mode, and sure enough, it will decompress a file from the Command window. Here is how to do it with JSL from within JMP.
RP = Run Program(
Executable( "C:\Program Files\WinRAR\WinRAR.exe" ),
Options( {"e -or C:\1\abc.7z C:\2"} )
);