There is no direct conversion tool or method. Create a new drawing from an Imperial template and then insert and explode your source file. You'll probably have to go through it and update or replace all of the metric styles with their Imperial equivalents.
Under the assumption that your drawing is say metric state plane coords, you can use Map to pull the data into your impearial drawing see to US survey foot in the same coord system. otherwise just insert / scale the drawing to convert from metric to imperial.