Measured against upcoming cross-border debt repayments, the additional SDR allocations could provide meaningful support for Zambia, Suriname, Tajikistan, Pakistan and Namibia.
Okamoto told reporters following IMF and World Bank Spring Meetings that the allocation, backed last week by G20 countries, will especially aid middle-income countries that need to refinance debt amid potentially tighter financial conditions.
Britain - which is chairing the Group of Seven (G7) this year - said G7 finance ministers had agreed to support a "new and sizeable" increase in the volume of Special Drawing Rights (SDRs), an internal currency used by the IMF.
Any expansion of SDRs will need to be agreed with other countries ahead of the IMF's spring meeting that takes place in April.
Both agreed that a new SDR allocation could form an important part of a package of support for low-income countries and could be vital for securing a truly global recovery, the statement added.
"We finally last Friday at the G20 meeting got a green (light) to work on a new SDR allocation," Kristalina Georgieva said during the IMF's African Fiscal Forum, broadcast online.
"500 billion - in which each and every member of the IMF would receive its own share immediately contributing to reserves," she said, without specifying the currency unit.