I modify Northwind.dbml file in WebGrid samples project by adding a stored procedure in the database. Right click the "Stored Procedure" folder in Server Explorer; create the SP; and save it.

Select ten most expensive products.

ALTER procedure "Ten Most Expensive Products" AS
SET ROWCOUNT 10
SELECT Products.ProductName AS TenMostExpensiveProducts, Products.UnitPrice
FROM Products
ORDER BY Products.UnitPrice DESC

To consume the SP, drag and drop the stored procedure (from the Server Explorer) to the right pane of your dbml designer.
This will create following method in the designer file.

[global::System.Data.Linq.Mapping.FunctionAttribute(Name="dbo.[Ten Most Expensive Products]")]
public ISingleResult<Ten_Most_Expensive_ProductsResult> Ten_Most_Expensive_Products()
{
	IExecuteResult result = this.ExecuteMethodCall(this, ((MethodInfo)(MethodInfo.GetCurrentMethod())));
	return ((ISingleResult<Ten_Most_Expensive_ProductsResult>)(result.ReturnValue));
}

I'm using OnSelecting event of ISLinqDataSource to call the SP and populate the result.

The aspx code.

<ISWebGrid:WebGrid ID="WebGrid1" runat="server" Height="300px" UseDefaultStyle="True"
    Width="400px" DataSourceID="ISLinqDataSource1" DefaultStyleMode="Elegant"
    OnPrepareDataBinding="WebGrid1_PrepareDataBinding">
</ISWebGrid:WebGrid>
<ISLinqDataSource:ISLinqDataSource ID="ISLinqDataSource1" runat="server"
    ContextTypeName="NorthwindDataContext" OnSelecting="ISLinqDataSource1_Selecting">
</ISLinqDataSource:ISLinqDataSource>

The aspx.cs (code behind) code.

protected void ISLinqDataSource1_Selecting(object sender, LinqDataSourceSelectEventArgs e)
{
    NorthwindDataContext context = new NorthwindDataContext();
    e.Result = context.Ten_Most_Expensive_Products();
}

protected void WebGrid1_PrepareDataBinding(object sender, ISNet.WebUI.WebGrid.DataSourceEventArgs e)
{
    if (!IsPostBack)
        WebGrid1.RetrieveStructure();
}

When the page rendered in browser, the grid will look like following.

Hope this helps.