Technically, yes that should be possible.
In practice, I'm willing to bet that the BT database will show you as having "an internet connection" on that phone number, which will prevent any ISP adding an order. Yeah, sure the internet connection will be via fibre, leaving the copper free for an xDSL signal, but the database might not know that.
Remains to be seen, I can't back this speculation up at all, but... you heard it here first