Can I add seasoning to the beurre manie?
Using roux - flavoured with curry powder and aromatics - is a well known, often used technique for japanese and chinese style curries. Your mileage may vary with beurre manie (known to be finicky), if results are not satisfactory try cooking the powder/aromatics in oil/butter and only then adding flour and whisking (classic roux technique).

Mind that indian and thai style curries use combinations of nut and seed pastes, coconut milk, yoghurt, cream, or just plain rich onion sauces/tomato stews to achieve thickness, sometimes helping things along with small amounts of cornstarch slurry - but rarely ever with roux and related techniques.