Manually adding books to Nook App on iPad
The Nook App isn't very intuitive when it comes to telling you how to sideload books onto your iOS device, however it is possible.  Unfortunately, it's not as simple as just hooking up the device via USB and letting Calibre handle it.  There are two methods:

Via USB (cable) and iTunes
Downloaded from shared site

USB Cable &amp; iTunes
It's fairly easy to drag and drop via iTunes.

Connect your iOS device
Under File Sharing
Drag and drop your eBooks to the Nook folder


Downloaded onto iOS device
This requires the Files App from Apple to access the folders on the iOS device

Download the eBook using your browser (Firefox shown here)
Go into the Downloads directory
Long press the file to bring up the context menu and select Move
Select the Nook directory
Press Move