Niether Power Store or eCart offer the ability to import an XML file to your database.
In PHPMyAdmin and most other MySQL GUI Tools, there is an import option that will allow you to import XML or CSV data to your database.
I found this page through Google that has details on the step[s for impoertting CSV data using PHPMyAdmin:
phpmyadmin-import-excel-csv-into-table/
There are many other such tutorials