No, battery thingy would likely not work. There were instructions on how to make a TSU1000/2000 RF-capable many years ago. Search since dawn of time. Specifically, you had to drill hole in your case and purchase an RF transmitter which, for US would have to be 418MHz or 433MHz depending on equipment you wanted to control. May want to also search for keywords +Dave +Houston as I think he contributed to some of these posts.

Keep in mind that even though you do this, your classic pronto (if not a TSU6000/Marantz 5200/9200) will send both RF and IR at same time as software was never designed to send RF or IR based on activity/configuration setup. All that work and still, you are stuck with a bastardized remote that cannot differentiate between IR and RF for a specific set of IR codes.

My suggestion? Prontos are cheap these days. Pick a 6000/RC9200 up on Ebay or heck, even pick up a TSU7000/7500 as these are ALL RF capable and talk to your US 418MHz extender. You can purchase the RUXXX equivalent (if you can find one on Ebay) and it will talk to a UK Extender @ 433MHz.