So you're talking about importing from a non-Zen Cart database (if I understood correctly)

If it has all your product info in one table - then your best bet is probably the Easy Populate add-on for Zen Cart (which uses a specially formatted CSV file to populate your products).
If I remember correctly, the installation for this mod should also create a sample csv file for you, so go over this file and check how it should be structured.

Then, use phpmyadmin to export your old products table from your existing database into a CSV file

Using Excel or any other spreadsheet tool, it should be relatively easy for you to reformat this CSV file for EasyPopulate's required layout (column titles, column order, adding any missing columns etc), then import this file via EP.

Good luck