Yes, but not in a way that nesmaker is set up to support natively. There are ways to hijack the monster chr files, but it does not replace the player game objects CHR in memory. So if you're using monster CHR to draw your player, your actual monsters will have be fit into the same graphics sheet, which provides a multitude of limitations....
So short answer, yes, witch clever coding and space saving measures.. Long answer... it's complicated.... You would need to change a great many things to get it operating.