item
is an instance of the ComboBox class. Hence it is a control. It is not a list item of a particluar ComboBox instance.
You don't need to create a ComboBox object , if you just want to add the list items. Same logic applies to any control.
You can try somethimg like this.
List studList = dm.getStudList();
foreach(Student s in studList)
{
ListItem item = new ListItem();
item.DisplayMember = s.getStudentName();
item.ValueMember = s.getStudentId().ToString();
cb_StudentName.Items.Add(item);
}