About 2.5% of the planet's water is freshwater; about 99% of that freshwater is ice, so considering only freshwater ice, if it all melts we'd have about 2.5% more CO2 capacity. There's also such a thing as seawater ice (see: North pole), but it doesn't add that much volume to the system, overall.
Any expansion in CO2 capacity due to melting ice will be completely dwarfed by other effects.
Without knowing how much CO2 is stored in the sea it's difficult to say whether 2.5% more CO2 capacity is much or very little.
I think the key question is: How much methane/CO2 is a liter of water in the state of ice storing and how much is the average liter of sea water storing.
Only the first half of this page is devoted to stating facts; the latter half is a call for political action. The facts section states that the millions of tons of methane trapped in the Arctic ice are a factor not commonly taken into account in future climate models, and its release will very quickly worsen global warming to the point of posing a serious existential risk.
http://www.alternet.org/environment/giant-methane-monster-lurking?paging=off