CU*BASE does require a number to be entered, but you can create a membership using an imitation SSN.   See the related links below for guidelines and tips.

As far as tax reporting, CU*BASE cannot validate SSN/TIN information.  Your credit union is responsible for providing accurate information for tax reporting.  Since we do not hear from the IRS on these situations, our assumption is that the IRS will treat imitation SSNs/TINs the same as any other invalid number and correspond with your credit union directly, possibly requiring you to set up backup withholding.