Yes, your existing Meta audiences will be available in Bïrch post boosting and Bulk creation given that they are added to “Saved audiences” category on Meta.

To do this, open Meta Business Manager, click on Audiences on the left side menu, then click on the "Create Audience" dropdown and select "Saved Audiences" from the bottom of the list.

Once the audience is added to the Saved Audience, it will appear in the Bïrch interface.