Hi,
I'm not sure how adding fields like that to your RSS feed are critical to your business since such extra fields do not exist in the RSS specification and thus won't be understood by any RSS reader:
cyber.law.harvard.edu/rss/rss.html
It's technically possible to add such fields in a RSS feed but RSS readers won't read them so there is no point.
And since it's not something standard, the Joomla feed system doesn't support that either, so before adding extra fields like that in HikaShop, you would first have to modify the Joomla feed system in order to support them.
In short, I don't see the point and I don't recommend such modifications.
If you want to add extra data to the RSS, then simply append it to the description of the $item object like that:
$item->description = $description.'<br/>Company:'.$company;