The best way to get/download apk files is ASTRO File Manager.
1. Download Astro from google play FREE on your android phone
2. In Astro! click Application manager and choose app you want to backup
3. Send app from you phone by mail, dropbox, AirDroid .....etc
That is all
sorry, but this isn't the "best way".
First you have to look at the motivation of this extension:
It is for NOT using the play market. As for people, woho don't want the spywa...google products on their device.
This said, your solution is not possible.
if you just want to get your apks out of your phone.. this is an easy one, as there are many backuptools in different kinds and shapes, which mostly are able to do this.