There is an example on the following page in the IDL documentation that hows one can take a normal image file, read the image data into IDL, and then write the data into an DICOM file:
http://www.harrisgeospati...x__setpixeldata.html
IDL does not have support for DICOMDIR. Harris GeoSpatial has a feature request in our system (IDL-41994) for DICOMDIR support to be added.