I have an old Mavic comete single speed rear track wheel. Can it be modified to accept a 9speed Shimano cassette or freewheel?

An old track wheel might take a freewheel instead of a track cog and a locking ring. But I think the cog threads are pretty narrow and the locking ring is another size thread. So it might take a multispeed freewheel but it would not take a cassette. As I understand it all the modern wheels over the last several years use cassette hubs so your track wheel is not likely to take a 9 or 10 speed cassette.

However, it might be just the kind of hub you could sell or trade to get what you want. I bet the single speed folks would fall all over themselves to get it.

So how much do you want for it? :)