If your employer is making you perform some "volunteer work" with the threat of termination or some other adverse employment action, then you may have a claim for wages if the employer is not paying you for this time. Federal law requires you to receive a minimum wage for every hour worked. If the additional work means working more than 40 hours in a workweek, then you may be entitled to overtime pay (1.5 times your regular hourly rate), unless you are in a position that is exempt from the overtime pay requirements. Wisconsin law has similar provisions for minimum wage and overtime, and does require payment of your wages within 30 days of each workday. You can bring this practice to the attention of the employer by talking or writing to a higher level supervisor, someone in Human Resources/Personnel or someone designated by policy to receive internal complaints about this practice. You can also file a complaint with the Wisconsin Wage & Hour Bureau and the Wage & Hour Division for the Federal Department of Labor for investigation. If you are not getting paid the minimum wage or overtime wage and your complaint refers to these unpaid wages, you may also be protected from retaliatory actions for bringing a complaint.
If you mean volunteer - as if work for no pay, then no. An employer cannot require you to volunteer your time. Under the Fair Labor Standards Act, you must be paid for all hours suffered or permitted to work.
If you mean the employer is requiring that you volunteer outside of work time and, if you don't, you will be fire, then it depends on the state and specific facts of your case. Keep in mind, an employer can make certain duties part of the job requirement.
I suggest you contact an attorney in WI or contact your state Department of Labor.
**This answer is not legal advice, but a casual response to an inquiry with very limited information. No attorney-client relationship has been created by my post.**