How to programmatically add a custom field to Address? I'm using Address module, and I need to add a Boolean field "Handicap access" for all addresses, whichever country is selected. 

I digged into module issues, I carefully read provided docs, where there is an example about how to : Create a custom address field by repurposing an unused one. 


  What about address field X? Can I add another property to an address
  field?
  
  No, not without great difficulty. But you can repurpose an unused
  property for a custom one.


But there is not any boolean existing field into Address fields. Any tip to put me on the right track is welcome. Thanks !
Not sure if this solves your specific problem, but you can create a custom paragraph type with an address field and a boolean field, then add a Paragraph Entity field on your content type. You can limit this paragraph reference field to your newly created paragraph type. This will allow you to have a field with both address/accessibility information.