Hey there,
You should be able to use Leads->Emails in Workflows
As Emails is a Module on its own, which is related to Leads, you’d have to select it as a related module
ie:
However, I don’t think you’d be able to compare the Email Addresses of both Leads and Contacts through Workflow
Workflow doesn’t currently have cross-module capability to that extent, i’m afraid 
Though, if you feel comfortable with PHP, you should be able to achieve this through a Logic Hook!
I’ve written some brief steps on creating Logic hooks here:Change user when isn't administrator
As well as there being the Logic Hook section on our Documentation: https://docs.suitecrm.com/developer/logic-hooks/
And further information on manipulating CRM info: https://docs.suitecrm.com/developer/working-with-beans/#_get_list
With a Logic Hook, i’d imagine you should be able to get all contacts where the Email address matches the submitted Lead
Let us know if you need any assistance or information!