How to write unitTest for methods using a stream as a parameter

Use a MemoryStream. Not sure what your function expects, but to stuff a UTF-8 string into it for example:

//Act
using (var test_Stream = new MemoryStream(Encoding.UTF8.GetBytes("whatever")))
{
    var result = imp.Import(test_Stream);

    // Assert    
    Assert.IsTrue(result);
}

EDIT: If you need an Excel file, and you are unable to read files from disk, could you add an Excel file as an embedded resource in your test project? See How to embed and access resources by using Visual C#

You can then read as a stream like this:

//Act
using (var test_Stream = this.GetType().Assembly.GetManifestResourceStream("excelFileResource"))
{
    var result = imp.Import(test_Stream);

    // Assert    
    Assert.IsTrue(result);
}