I don't currently participate in a 401K at work because they only offer a Roth...
You may want to obtain a copy of the documents governing the plan, as is generally your right under § 104(b)(2) of the Employee Retirement Income Security Act of 1974 ("ERISA"), PL 93-406, 88 Stat 829, 849, codified at 29 USC § 1024(b)(2). An employer is not required by any law to offer a plan that meets the requirements of § 401 of the Internal Revenue Code. However,
assuming that the plan actually does meet the requirements of 26 USC § 401, then the law contemplates that, if the employer offers a "qualified Roth contribution program", the employee "may elect" whether contributions are treated as normal elective deferrals or rather as "designated Roth contributions". 26 USC § 402A. The IRS website is not the law, but it explains this in much simpler language: "[
I]n order to provide for designated Roth contributions, a plan must also offer traditional, pre-tax elective contributions".
IRS Retirement Plans FAQs on Designated Roth Accounts.