Serious pack rats might soon be deemed mentally ill under a proposal to make compulsive hoarding - a condition that may afflict up to 300,000 Canadians - a new brain disorder.Involving everything from old papers and magazines to empty yogurt cups and toothpaste tubes, compulsive hoarding is described as the excessive acquisition of, and failure to discard, what most would have tossed out long ago. Hoarders are convinced the material might be useful or important some day.