Hey
If you have a proprietary system that it will come down on how you can access it or generate data.
If you are able to generate csv files and ftp them to a location or provide ftp access to them there are many apps that will capture and import to Shopify.
We normally create bespoke solutions where there are APIs available to integrate to complete data flows for our users.
A question would be if moving to Shopify what can you do on your proprietary system that you cannot shift to Shopify and simplify your processes?
Regards
Tom