Jenkins pipeline emailext emailextrecipients: Can I also add specific, individual email address?

I don't know how I missed this, but the answer is in the email-ext doc. Use the to: for the additional email addresses, and use recipientProviders: instead of to: emailextrecipients. So one would have:

emailext (
    subject: email_subject, 
    mimetype: 'text/html', 
    to: '[email protected]',
    recipientProviders: [[$class: 'CulpritsRecipientProvider'],[$class: 'RequesterRecipientProvider']], 
    body: email_body
    )

A slight variation to Generic Ratzlaugh's answer, in case you need to use conditional logic for email destinations.

def myProviders = [ [$class: 'CulpritsRecipientProvider'], [$class: 'DevelopersRecipientProvider'] ];

myProviders.add ( [$class: 'RequesterRecipientProvider'] );

emailext (
    subject: email_subject, 
    mimetype: 'text/html', 
    to: '[email protected]',
    recipientProviders: myProviders, 
    body: email_body
    )