Hello ,
Thank you for posting your question in the community and this feature to export the layer/channels is not possible at this time. You can make a comment on the road-map as suggested by @嗣人 西脇 .
Here is a discussion that might help in regards to creating a white background.
https://twinmotionhelp.epicgames.com/s/question/0D52L00003XQ0VJSA1/how-to-export-render-a-png-image-without-background-in-twinmotion
Alternatively you can also use this trick to help create an alpha channel type of image by using a skydome object or background plane.
https://www.facebook.com/groups/twinmotion.community/permalink/813601109531185/
Example: "pink skydome, ambient to 0, play with contrast and exposure. set skydome to translucent or emissive - it will depend on the scene"
Hope this helps.
Kind regards,
Vincent B.