Let me provide some information about setting up workers comp classes for an employee, @beckycox.
I recognize you need to assign multiple classifications for your employees. Currently, this option is unavailable in QuickBooks Online.
At times, we roll out enhancements based on our customer's requests. That being said, I suggest sending a recommendation to our product developers. They'll review this idea and might consider updating it in the future.
Here's how:
- Go to the Gear located at the top.
- Under Profile, click Feedback.
- Enter your recommendation and select Submit.

You can also utilize this link to check the status and keep you in the loop on our progress of your request: Customer Feedback for QuickBooks Online.
I also encourage you to visit this resource that will guide you in setting up, change, pay and file WA workers' compensation: Manage Washington (WA) workers' compensation.
As always, don't hesitate to leave a comment below if you have any other questions about adding workers comp classification in QuickBooks. I'm here to help always. Take care!