Interspire Shopping Cart 5.5 and newer does not contain a specific store importer function for any third-party carts. You may use the product and customer functions built into the software to import this data from your existing site.
For more information, refer to these articles:
How do I import products with a CSV file?
How do I import customers from a CSV file?
How do I import digital product files when importing products?
How do I import images when importing products?