Custom Auto-generated username

WordPress Community,

I’ve finally made the transition from Joomla to WordPress but could use some help. Thank you in advance for your help!

I’m using WordPress 4.5.3 with the Woocommerce plugin. I would like a system generated username but not the one generated by taking the email address minus the domain name. Instead, I’d like a unique identifier that looks like this SK00000 and auto increments for each user (I know this is not super user-friendly but its a legacy requirement). Can somebody guide me on the best way to do this? It looks like I have to modify the modules specific to wordpress as that’s what appears to be generating the username. Is that correct?

-H