Could you please confirm that you are trying to include the signature inside the conversation inbox? If so, currently the conversation inbox is "shared" and HubSpot does not allow the setting of a universal team email signature.
If your goal is to have each individual user's signature from their settings go into the shared inbox, that is not part of the original idea post and will need to be upvoted on in this Ideas forum. The progress on that capability that can be tracked there!
If it's not in the conversations, could you please provide us more details? The more information, screenshots, and details you can provide, the better I can advise on the next steps.