order.htm
states:
WinRAR, RAR for Unix and OS X now use the same registration key format, so you can use the same key with current WinRAR and RAR versions for all mentioned platforms.
então sim, é possível.
Além disso, também afirma onde copiar a chave:
If you use RAR/Unix and RAR for OS X, you should copy rarreg.key to your home directory or to one of the following directories: /etc, /usr/lib, /usr/local/lib, /usr/local/etc. You may rename it to .rarreg.key or .rarregkey, if you wish, but rarreg.key is also valid.