Hi,
If you DataBind() your list before inserting the new ListItem you should find it will display as requested.
Something like this should work:
ddlPromoSuffEdit.Items.Clear();
ddlPromoSuffEdit.DataSourceID = "";
ddlPromoSuffEdit.DataSource = odsPromoSuffEdit.Select();
ddlPromoSuffEdit.DataBind();
ddlPromoSuffEdit.Items.Insert(0, (new ListItem("All", "")));
... hope it helps.