Older pocket tv's will pretty much be useless once the changeover occurs. You need power for a box. Most pocket televisions only have a simple metal rod antenna. You cant hook up a coax or anything like that.
You MIGHT be able to use a UHF/VHF balan to do it.
If the tv has the screw connections just get a balan and screw it in and then hook up the box as you would on a normal tv.
Balans are about less then 2 bucks at any radioshack.