I'm afraid we are not Twinmotion experts but we are going to run some tests and see what's possible. In the meantime our usual recommendation would be to use "Forest Tools". This allows you to export the geometry as instances. It can be found it under Utilities panel - More ... - Forest Tools. For more details and available options, please check our online reference guide
Once "intantiated", I would suggest you export these instances in the .FBX format. When exporting, please feel free to select the FBX Export window - Geometry, the Preserve Instances option, though I'm not sure if or how this is currently implemented in Twinmotion - as I can see, there's also a Collapse > Keep hierarchy option which might also be worth experimenting with.
I hope it helps you, we'll report back here once we've tried some more tests.