Code injection

Im needing to create a postinst script for a debian package. I need it to search for this line of script in a plist file located at /folder/folder/folder/folder/folder/file.plist

<key>TESTKEYLINE</key>
<true/>

and after it create a new line and insert this code

<key>KEYNAME</key>
	<array>
		<dict>
			<key>KEY</key>
			<false/>
			<key>SUBJECT</key>
			<string>SUBJECT NAME</string>
			<key>SUBJECTIMAGE</key>
			<string>IMAGE.png</string>
			<key>IMAGESETTINGSBASEDONCONDITON</key>
			<string>2NDIMAGE.png</string>
			<key>SUBJECT</key>
			<string>SUBJECTNAME</string>
			<key>IMAGESETTINGSBASEDONCONDITON</key>
			<string>3RDIMAGE.png</string>
		</dict>
	</array>

After the code is pasted on the plist file I need it to require a reboot.

Sorry about the vagueness of the code, but I don't want this being in someone else's hands.