Edit your plan then under Integrations > Options > Prices you can configure additional price/duration combos the same way you would under the Pricing tab of your plan. See screenshot below for guidance.
Once there be sure to name your price option. Something like 1-yr for example would be fine. Next set "Regular Price of plan" as desired and "Regular Validity duration" to "1 year" (should already be defaulted to 1 year). Don't forget to enable price options at the top as well as that's a global on/off toggle for plan pricing for that plan.