This feature is currently not included into the API and we cannot integrate with it.
We keep our finger on the pulse and as soon as it will be available in the API we make sure that it will be available in signaturky.

Multiple signatures is the feature that allows to set up different signature for the new emails and emails that are reply or forward. You can read more about it in dedicated Google help page.

GMail settings for Signature defaults - set different signature for new emails and on reply/forward