The proposal to include lower earners in the Government's pensions auto- enrolment scheme would include more than one million lower-earning women who Labour say have been excluded from the project.
In a speech to the Resolution Foundation, Ms Reeves was to say the rise in zero-hours contracts, part-time and low-paid jobs means more people risk missing out on the chance to save for retirement.
Labour estimates the proposal to lower the threshold for auto-enrolment from the current £10,000 to the national insurance lower earnings limit of £5772 will cost £20 million by 2018/19 due to extra pensions tax relief.
The party is examining plans to pay for the proposed scheme by cutting the cost of appeals against fitness-to-work assessments, which cost the Department for Work and Pensions £30 m last year.
Ms Reeves was to say: "For far too many people the insecurity they face during their working life is set to continue when they retire.
"The Government's failure to encourage more people to save threatens to store up huge costs for taxpayers in the future with a rising benefits bill."
Neil Carberry, CBI director for employment and skills, said: "Given we are half-way through the roll-out of automatic enrolment, changing the rules again is unlikely to assist businesses or workers. It's not clear that automatically enrolling this group is necessarily in their long-term interests."