Hi,
Well, you can directly update the tables of the database with the data coming from your external system.
So it's possible with a custom script.
That script can either be in the form of a plugin of your external system which would update the data in your website database, or a Joomla plugin which would be triggered periodically (for example with the HikaShop cron trigger) and which would check the data from the external system (by accessing its database or some kind of export) and then updating the data in the database.