If you want this to happen automatically then there is a lot of work involved, and will probably require some expert coding.
It is unlikely that the feed coming from your drop shipper matches the database table and field structure for zencart, so before it can be imported (even manually) it will have to be "re-organised" so that it matches zencart's database structure.
Then, once the data is configured properly, a second set of code files will go about "importing" that data into the database.
But getting the data into the database is the first step... You are also going to have to look at how it is managed.
For example... you will also need feeds that alert you to out-of-stock products at the drop shipper, and feeds that alert you to discontinued items... and feeds that alert you to "items that were out of stock but are now back in stock"... and feeds that alert you to just new items... and feeds that alert you to modifications to existing items (new price, small change in size specification, etc).
We had a client in Spain who wanted this, and we got a Russian coder to build it. It wasn't cheap...