How about using the existing as a "rug" and doing a border ( not herringbone, brick pattern) around the rug using new parquet. Any existing "border" parquet could be moved and used to extend the 'rug' into the extended hallway. this way, differences of size and wood colour could be accommodated.
I have no idea about how'd to lay it and sorry it out to match. We're looking at using reclaimed parquet though and there seem to quite a few suppliers. We had a look in Lassco, www.lassco.co.uk I think. they may be able to provide advice on what to do.
I had herringbone parquet flooring years ago, made from individual wood blocks. One piece had woodworm and needed replacing - sourcing one block in the mid 1990s was hard enough! I think ours was teak (house was 1930s). In the end we took the wormy block to a wood merchant and asked them to cut a new block, not from teak though (iirc), but a similar hardwood. Yours may be teak as well - it's cheaper than mahogany.
I really wouldn't lift the parquet you already have, to be fair, especially if you're on a tight budget - I understand your desire to mix in the new with the old but you'd be better off trying to match the wood as closely as possible, then sanding it all back and re-varnishing the lot.
If you can't find actual single parquet blocks, you might be able to get patterned "tiles"/panels of parquet that will match closely enough what you already have.
Our extension means a longer hallway. I'd love to keep the parquet and extend it. How easy would it be? I presume I'd buy reclaimed, matching-ish parquet, lift the existing floor, mix it up and relay it?
I should add, we are on an impossibly tight budget after getting planning permission for everythign we applied for. Hadn't anticpated that - now no money for kitchen/2 bathrooms/decorating/flooring....