Assumption -- a segment represents a subset of a subscriber list - how is this acheived via the API?
a new subscriber list is created via CampaignMonitorAPIWrapper.List.Create
how do you go about creating a new segment for an existing subscriber list?
It appears as though you are using the old .NET wrapper which applies to v2 of the API, which does not support creation of segments.
This is one reason you should start using the current version of the API. v2 will eventually be deprecated. There is a .NET wrapper available for the current version of the API, which supports creation of segments.
All this information is available from the Campaign Monitor API page.