basically what Draftek said above, you can wrap existing classes or functions into managed classes for use with .net.
You can also call native code from your managed classes (this is how you wrap them after all), say you want to use winforms in mc++, you can call native functions from event handlers say all in the same module.
If you want to study up more on this have a search in the help doc's for 'It Just Works', this was done for this very reason, to integrate and help port existing native code into managed code.
There should also be a sample mixed arx project in the help I think, maybe if you include the necessary file you can do a 'go to definition/declaration' and see how some marshaling is being handled.
hth.