If you mean an over the air digital converter box it won't help you connect a Roku 2.
If you mean an HDMI to component or composite video plus audio converter then that will work. If the TV has component video get that converter as it may produce a better picture.
I would suggest that if you can exchange the Roku 2 for a Roku 1 that has composite video and audio outputs
https/www.roku.com/products/roku-1
that would be simpler and cheaper.