If you specify - as the filename, ximtoppm writes the alpha output to Standard Output and discards the image.
Actually, an Xim image can contain an arbitrary fourth channel -- it need not be an Alpha channel. ximtoppm extracts any fourth channel it finds as described above; it doesn't matter if it is an alpha channel or not.
See pnmcomp(1) for one way to use the alpha output file.
All options can be abbreviated to their shortest unique prefix.