The short answer is "no", at least in the spirit the question is asked.

A small diamond would occupy a volume of about 1 cm cubed, and weigh
 in the region of a few grams, so to fill a large room, the average
 density of the dust would need to be more than about 500 cubed (125
 million) times less.
Diamond dust CAN be, however, very fine, and consequently could be
 completely and permanently dispersed within the air in the room by
 tiny updrafts of air etc. In this sense it could be said to "fill"
 the room, even though the total amount present has not changed.