How InfoPath data is stored in database? Is it XML or some other format? Can I import excel data to an existing InfoPath web template?
An InfoPath form that is not based off a database cannot be submitted to a database out of the box. InfoPath forms generate XML files. These are often submitted to a SharePoint form library, but can also be saved other locations.
If you know the schema for your form, you could generate XML from an Excel file - there are third party tools that can do this (note, I work for a company that sells one) or you can write your own....
I think depending on the complexity of the schema, you could probably even use a somewhat complex Excel concatenate formula to create the XML string and just paste each result into a new file in a text editor and save as XML.
If you take a look at an InfoPath form's XML in a text editor, you will see there are processing instruction in it that reference the form template and indicate the file should be opened with InfoPath.