I’ve used Mimecast to do this for free https://community.mimecast.com/s/article/Stationery-Layouts-Inline-Signatures-1998822414 , there are some limitations
Tag: signature
How to Use Powershell to Mail Merge Outlook Signature for Users out of Active Directory
Script 1 Script 2 Script 3 https://github.com/raymix/PowerShell-Outlook-Signatures