And there is no way to put my main domain com.ar in my cpanel¿? you can not changed?You can't. Users are no longer permitted to change their main domain. You can add your domain as a Parked Domain, which will have pretty much the same effect, except that if anything happens to your external domain (like your registration expiring, etc.) your site will still be accessible using your x10Hosting domain.
No, you don't "need". Add your domain as a parked domain, and it will work.