<select>
<option value="1" data-value="myValue1">item</option>
<option value="2" data-value="myValue2">item</option>
<option value="3" data-value="myValue3">item</option>
</select>
<select id="selectList" class="multiselect" multiple="true" name="selectList[]" runat="server"></select>
ListItemCollection values = new ListItemCollection();
ListItem test = new ListItem("add");
test.Attributes.Add("data-value", "myValue");
values.Add(test);
this.selectList.DataSource = values;
this.selectList.DataBind();