Well, you've actually got exactly the statement you need to do what you want. The tricky part is getting your hands on the selected item!
Here's a sample script that will append a string to the notes field of the selected items. Note that it does not drill down into contained items.
Code:
tell application "OmniFocus"
tell front document
tell (first document window whose index is 1)
set theSelectedItems to selected trees of content
end tell
repeat with i from 1 to count of theSelectedItems
set selectedItem to value of item i of theSelectedItems
set theName to name of selectedItem
set theNote to note of selectedItem
set theNote to theNote & "
This is a note inserted in " & theName
set note of selectedItem to theNote
end repeat
end tell
end tell