Is there a way for a customer in Queue to dial an extension to be sent to voicemail?

yes, I will share what I think is the best way to do this.
Go to Phone System > Groups > Message - only extension.
Create an extension that will be receiving the voicemails. Think carefully about the ext. number - you want it to be easy-to-input, and at the same time different from all of your other extension numbers so that other extensions don't get accidentally dialed. Consider different ext. number length options - 1-digit, 2-digit or 3-digit.
Configure the voicemail greeting and the recipient/email notifications for whenever somebody does leave a voicemail.

Then go to the call queue > Greeting and you want to use either Call queue greeting or Interrupt Audio, to inform your callers of the possibility to leave a voicemail. For example you could set up an Interrupt Audio message that plays every 30 seconds that says: "if you do not wish to wait, press <xxx> to leave a voicemail" where xxx is the ext. number of the Message only ext.

Some considerations regarding the numbering
- 1- digit ext. numbers are not recommended because of the small chance somebody presses the key on accident. This could also be an issue in rare situations where you have the Message only ext. in an IVR Menu which has keypresses 1-9. It's a good idea to not have other 1 -digit extensions such as call queues, so that calls don't get accidentally misrouted to another call queue. The benefit of 1-digit ext. number is that it is the easiest for the caller to input.

- 2 - digit - pretty easy to input by the caller and 0 chance of dialing the message-only ext. on accident

-3-and-more-digit length - gets more cumbersome to go to voicemail.

I encourage you to try out 1- or 2-digit ext. number length!

Thank you! That's exactly what I was looking for!