If for all the emails, you want to set the Regarding as the "From User",
1. Create a workflow that will be triggered on create of email.
2. Set the Regarding as the From user and update the record.
3. Modify your existing workflow to either trigger on Email update or add your current workflow logic in the one created above.
Note: This would not be an ideal solution if you do not want this to happen for all emails. You will need to filter out the email in your workflow created (in step 1 above), so that it would work only those emails that you desire to.