Yes but possessing the Heart of Grima is completely different from being able to use it. When Lucina used the Gate to travel back into time she was followed by Grima's spirit who attempted to possess his vessel of that time period and give it the memories of what is to come. The Avatar at the time had a much weaker Heart of Grima and so lost both his memories and the abilities to utilize the Heart. Hense why the Avatar can't use dark magic in game.