As per the title can't see if this is possible, I'm chaining cars and roof bars soon and new bars will need t rails on the bike carriers.
I've just swapped cars and therefore roof bars. Thule have the T Slot bolts available to convert their bike racks to fit T Slot. Google search it. I can't remember which UK site have the kits in stock.
They weren't cheap though.
Mine came with all the bits you need to go either T-Bolts or clamps. Have you still got all the stuff that came with your rack. All you need is a T-Bolt. They look like they're £3.95 each...see link below. I don't see why any other T-bolt wouldn't work. I don't think they're special in any way.
https://roofracks.co.uk/Thule-Products/Spare-Parts/_/?productID=591
We have 2 different models of thule bike carriers. The newer model came with the track adapters, but the older ones didn't. I brought my bars and extra adapters from the roof box company. They have been really helpful with recent orders. I couldn't initially tell the difference between the adapters, but on closer inspection the threads are different. Therefore you do need to buy the correct ones to fit the winged nut. The ones I needed only cost £7.50 each.