It might not look like a quick fix but you could bring them in make them quarantine for 2-3 weeks then get them up to speed and into the system. The initial quarantine would be dull however it could be combined with Zoom/Teams recurrent AEP and other regulatory items.
At the moment hkg based crew are facing months of potential quarantine blocks so might as well have the based crew do it once get retrained then just operate loops of flying without having to quarantine again!