Here Maps has the hability to download maps in a very compressed format. Unfortunately, this format is a proprietary format, which means that only Here Maps can read those files. Applications such as Maps Pro won't be able to read them.
Also, Windows 8 Applications are "sandboxed" which means Maps Pro can't read from the folder dedicated to Here Maps.
It is possible to use the Nokia Here Maps basemaps in Maps Pro. In the bottom menu bar, choose "Map Styles" and then choose one of the styles from the "Nokia" section.
Then, navigate to the area you want to extract for offline usage, for instance Rome and in the menu, choose Offline > Download this map. Choose how many zoom levels you want to extract. It will decide how "deep" you can zoom in once you are offline and it will also affect how much size it takes on your hard drive.
Choose a folder where your files will be downloaded and then click "Download". It might take a while.
Once it is finished, you can try it going in airplane mode or trying the option "Use offline maps even when online" in the Offline menu.