Microsoft Outlook does not roam user's signature. It needs to be backed up manually before the user is switched to a new computer of a new Windows profile.
1. Sign into Windows using credentials of the user whose Outlook signature you will be backing up.
2. Launch Windows Explorer.
3. Type the following path in the address bar: C:\Users\userid\AppData\Roaming\Microsoft replacing "userid" with user's actual userid
4. Locate a folder called Signatures and make a copy of it to user's OneDrive.