You can open the list in datasheet view (or create a datasheet view) and just copy & paste your excel sheet rows into the list. Just be careful that the columns are in the same order as in your sheet.
If this is not possible, you need to use script or code. The problem here is to use the right format for your fields.
Example:
(src: http://social.technet.microsoft.com/wiki/contents/articles/18830.sharepoint-2010-import-data-from-excel-into-a-list-using-powershell.aspx)
#Get the CSV file and connect to the SharePoint list
$vessellist = import-csv -Path "C:\Temp\VesselInPortReport.csv"
#Get the list used to import the data to
$l = (Get-Spweb "http://test").GetList("http://test/Lists/yourlist")
#Get the lists EmployeeType field (choice)
$employeeType = $l.Fields["EmployeeType"] -as [Microsoft.SharePoint.SPFieldChoice]
#Loop through the items and add them to the list
$r = 1;
$itemCount = $vessellist.Count;
$currentItem = 1;
foreach($item in $vessellist)
{
Write-Progress -Id 1 -ParentId 0 -Activity "Importing Data From CSV into SharePoint" -PercentComplete (($currentItem/$itemCount)*100) -Status "Adding item $currentItem or $itemCount";
$currentItem++;
$ni = $l.items.Add();
#Add the Title, using the rows VESSEL_NAME column
$ni["Title"] = $item.VESSEL_NAME;
#Add the "Date Recorded" field, using the csv rows "RPT_DATE" column
[DateTime]$rd = New-Object System.DateTime;
if([DateTime]::TryParse($item.RPT_DATE, [ref]$rd)){
$ni["Date Recorded"] = $rd;
}
#Add the csv rows "TRIP_NO" column to the new list items "Trip Id" field (SPFieldNumber)
[Int64]$tn = New-Object System.Int64;
if([Int64]::TryParse($item.TRIP_NO, [ref] $tn)){
$ni["Trip Id"] = $tn;
}
#Add some other text properties
$ni["Flag"] = $item.FLAG;
$ni["Agent Name"] = $item.AGENT_NAME;
$ni["Current Location"] = $item.CURRENT_LOCATION;
#Add user information
$ni["employee"] = $w.EnsureUser($item.EMPLOYEE); #In this case, the $item.EMPLOYEE value from the spreadsheet is a persons name. Eg. "Matthew Yarlett"
$employeeType.ParseAndSetValue($ni,$item.EMPLOYEE_TYPE); #In this case, the $item.EMPLOYEE_TYPE value from the spreadsheet is valid choice present in the EmployeeType list field. Eg. "Manager"
#Update the item
$ni.Update()
$r++;
}
I recommend just creating a new list from the sheet, it's less painful.