Sorry, you can't. By law, a 401(k) is an employer-sponsored plan and only the employer can make contributions (including employee contributions via a salary reduction agreement.) You may be able to contribute to a traditional or Roth IRA depending on your income, but those limits are lower, of course. (If you had lower than expected income for 2019 due to a job loss, you might have more eligibility to contribute to a Roth IRA than you originally planned. See here for the limits https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-ira-contribution-li...
