You can add a round remote to the Meta alarm at any time. In fact I have just bought another one from ebay. You will need you red code card, and you just provide that code whne you order the fob.
Abacus alarms do them for £35 and I got my last one from ebay for £25 =£10 for it to be coded. Service from both companies was excellent.
If you do nopt have the code card you can send your existing remote and alarm box and they will code a new round fob to it, along with your existing fob, and provide you with a code card too.
However I think the price for that exceeds that of a better modern alarm.