Looks answered, but yes you can.

Unlesss you are a SOT you will have to modify the bolt to semi specs as well as converting it from open to closed bolt. You will also need to add a blocking bar that will prevent a unmodified auto bolt from being dropped in.

The easiest way to do a Sten is to buy a parts kit and then order a reduced sized tube from Indianpolis Ordanace. They also have a modified bolt and firing controls for sale.