It is very difficult by yourself. However Marmalade have a product called Marmalade Juice that converts an app from Objective C to a marmalade format that can be used across various devices.
However, it will have many limitations, you must try it out first. I have used Marmalade but I have not tried Marmalade Juice, but I think you will have to rewrite some code to make it happen.
As for the advertiser API's, marmalade have many API's integrated in their codebase so you could use these instead of directly using the API's that you download from Chartboost. Some advertisers, such as Revmob provide Marmalade compatible API's directly.