How to add arraylist items to a unique stream

Question

Hi folks!

Well, take a look:
int nfiles = arrArchives.Count;
Stream st = null;
for (int i = 0; i <= nfiles; i++)
{
    st = arrArchives[i];
}

So, I want add arraylist items into "st" stream, how to do this?
This below code don´t works, the error: "Cannot convert source type 'object' to target type 'System.IO.Stream'"

Thanks in advanced :)