You might do this with EZ Populate (you shall have to install it).
You will have to adapt your input files (whether xls or csv) to the layout expected by EZ Populate.
In order to do that, just create one product in ZC (through Admin).
Then, Admin -> Tools -> Easy Populate.
Click on 'Download complete tab-delimited.txt file to edit'.
This will create a flat file on your PC. This flat file will have two lines : one for the column headers, another one for the only product in your catalog (created manually previously).
You can now update this flat file (best is to import in a spreadsheet program) with product data emanating from other sites.
When finished, save the spreadsheet as a csv file.
You then upload it with EZ Populate : Admin -> Tools -> Easy Populate. Provide the name of the file in the field 'upload EP file' and click 'upload'.
This process is valid if products are not priced by attributes.
If they are priced by attributes, then a 2nd process is needed for the attributes.