Is there a way I can move all the signatures I have under "Signature on this device" to the signatures I have under my mailbox as in roaming ones.

Hello Deepanshu_Gauba

Thank you for using Microsoft products and posting them to the community.

I realise that you want to migrate outlook signatures on a certain device and I understand how you feel.

I'm sorry to inform you that Outlook does not currently have a standard feature for migrating electronic signatures.

However, if you have a need to transfer signatures from Outlook on your old or new computer, I have found a viable alternative for you to try. You can check it out for more details: How to import and export signatures in Microsoft Outlook? (extendoffice.com)

Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology. 

1. Open a folder, and enter the %userprofile%\AppData\Roaming\Microsoft\Signatures in the address box at the top, and then press the Enter key. See the following screen shot:

2. In the Signature folder, please select and copy or cut the corresponding three documents and one file folder of each signature that you will export.

3. Just paste the copied files and folders into the corresponding folders on the new device and see if Outlook works properly on the new device.

I hope this information has been helpful. If you have any questions. Feel free to let me know if my steps or understanding are wrong or if there is any progress on the issue.

Best wishes

Forry.F - MSFT | Microsoft Community Support Specialist