There's an import  from excel  spreadsheet option when you go to create
a list  - it's grouped with the custom list creation options.

As far as I can see, the import  From Excel option is
available to create a new list. The original poster
wants to import data  into an existing  list.

The only easy  way to import  data from Excel into an existing  list is by
cutting and pasting the Excel Records into the data  Sheet View of the
list.

Of course you could also do it programmatically, but most people don't
rate that as an easy way.

In SP 2007, possibly take the list  into data  sheet (Excel) view, add the rows of new data, then go back to normal view of the list. Have not tried it because I can't even get the data sheet view working because we use an older verison of Excel.

Or, export the current SP list to Excel, update it with new data. Creata a new list with that spreadsheet and trash the original SP list. But if you have anything fancy going on in your list such as lookups, attachments they could get lost. It may also add another ID column on the import  that needs to be cleared out.