As far as I know this is not a default feature in the API available to mods, but this should already be supported when "Enable expressions in inputs" are enabled in the per-player settings for Cybersyn Combinator. Is this setting enabled and it still doesn't work? (It should default to enabled but it's possible it might be disabled if migrated from an earlier version at some point.)
Supported (case insensitive) prefixes are k, m, g, and b (same as g), matching the ones available in the native game UI.