Sorry yep it was a little confusing. I've helpfully had concrete answer from BT now that I can add them without a contract extension. Thanks all.
@benwiddowson You could if anything use the BT Sports Pass App if you're trying to watch BT Sport which is a rolling 30 day type app if I'm not mistaken? For that you'd need to use the BT app on your TV/smart device rather than the actual BT box though. From what I understand (correct me if I'm wrong), but with the BT TV Pro box at least, you can record from the EPG guide and any NowTV channels are in the Guide now. I'm not sure about other Now packages though (e.g. Entertainment) when it comes to monthly contracts outside of having a BT TV contract, but those channels do appear in the EPG Guide.
Edit: Oh you've sorted it out anyhow. So you were able to add it onto your existing TV box monthly? Good to hear!