Sure; Tee fittings are available e.g. https://www.ebay.co.uk/itm/UNDERFLOOR-HEATING-16mm-20mm-PEX-AL-PEX-BRASS-COMPRESSION-FITTINGS/321982235054 ...

I'd consider taking an existing room(s) that are less often used/less important (utility+wc?) or an existing room with two equal loops and teeing them together onto one manifold port, then setting the flow rate on that port to be the sum of what the two were before, then using the freed-up port for the new room so you get independent control of its flow rate..

..though all that said, buying another manifold with enough ports and selling the one you have (keep the pump, TRV, bleeder etc) won't exactly be the biggest drop in the ocean of extending a house. Aaand, don't forget that for all the faffing there are also kits available to extend an existing manifold: https://www.ebay.co.uk/itm/Polypipe-UFH-Brass-Manifold-Extension-Kit-PB12736/124513203876 - 40 quid is possibly dear for what you get when you consider that a manifold is usually about 15 quid a port, but the time it saves...