Using an external part to copy hole positions

Your post title mentions hole locations, but your description asks about using the shape of the original part. 

In sketch mode you can project the perimeter shape of the original part using the loop option. This will create a sketch that will fill in between the rectangle and the original part.

There are a bunch of ways to do this, but perhaps the simplest way would be a Merge/Inheritance feature that just copies everything in your part. After this, you add an extrude with your big rectangle, and project the outer perimeter into your sketch, to fill out the missing geometry.

Another way would be if you used point patterns to define the original holes. you can then use a Copy Geometry feature to copy those points and recreate a point pattern on the new part.

Or you could get creative and make a drawing with a Hole Table, then copy the coordinates from that table to create a PTS file, import that into a point feature and again use point pattern.

Or the same method, but instead create a table pattern with the Hole Table data.

There are a bunch more ways, if those methods don't work for you, for one reason or another.