You can't, at least using an SPD workflow. You could always as VS workflow, or write a custom workflow action in VS.
See more here.
Update (2 June 2016) : Although i was agree till yesterday that its not possible as its totally controlled by SharePoint, however i am pleased to share that we have resolved this issue through making tweak on exchange server side.
Problem Cause: The cause is the alert mails were relayed to Exchange Server anonymously. All the versions of Exchange Server do not try to resolve the sender of the message when message is sent anonymously while the recipient is always resolved as we have to deliver email to him .
If the message is sent to Exchange Server using authentication, Exchange would try to resolve the sender against an object in AD and change the Display Name if it exists on the object.
Configurations on Exchange:
Here are the configuration steps:
Create a Receive Connector using default settings.
Double click Receive Connector, switch to the Permission Groups tab, check the Exchange Server, click Apply.
Switch to the Authentication tab, check the Externally Secured, click Apply.
After configured the settings, you have complete trust the previously designated IP addresses.
Source of above update is described here which is actually implementing in MOSS 2007 but we have implemented the same in SharePoint 2013 and Exchange 2013 server. Which worked fine.
Hope this update will help some.