Yahoo Mail has a feature which lets user to add their signature as part of their outgoing emails. This can be done by using the Yahoo mail options.
Click the Options link available on the right hand side and select More options from the menu list.
This would display the following screen with various links. From the displayed links select Signature.
The following Signature screen would displayed.
To add a signature to your yahoo emails, select Show a signature on all outgoing messages radio option and enter the signature in the text box given below the radio option.
Click the Save changes to the confirm the changes.