How can I load images of type X into Slicer?

Your best bet is to convert the files to raw format (no headers) and read them in with manual header information. Check out imagemagick.org or IrfanView.com for a general purpose conversion tool, though the Slicer Development team does not support nor guarantee the use of these tools. The file numbering scheme should match the standard MR scan scheme (%s.%3d) for easiest importation.