Hi,
of course you can; there are several strategies, depending on the capabilities of the app you want to copy from:
1) If the app supports "Open in...", then chances are good that Notebooks is listed as an option
2) You might be able to export the notes in a standard format and then import those notes into Notebooks (you find more info about that in the handbook, available from http://www.notebooksapp.com/Download.html)
3) You can always copy the text of a note, switch to Notebooks and then either paste the text into a note in Notebooks (this is standard iOS functionality), or select "New from Pasteboard" from Notebooks' "+" menu
4) For importing from the iPhone's/iPad's Notes app, you find a detailed description on our website and in the handbook.
I hope this helps.
Best,
Alfons