Hi Ken,
Composer does not currently support chord diagrams, though that feature is on the to-do list. Composer also does not support pasting in of graphics.
I have done this the other way round though - using PDF Creator (
http://sourceforge.net/projects/pdfcreator/) to save my score as a jpg or other graphic filetype, and then pasting diagrams on top of that. (I was using it for tin whistle fingerings.)
ttfn,
Sherry