OK, I haven't tested this but it might work:
Mount and copy the contents of the Tails ISO. cd into the directory where the ISO contents were extracted. There should be a >800 MB file under live/filesystem.squashfs. Mount that file.
You'll get an error saying it's only been mounted read-only. That's OK.
sudo mkdir /mnt/tails-iso
sudo mount Tails.iso /mnt/tails-iso
cp -r /mnt/tails-iso ~/tails-dir
sudo mkdir /mnt/tails-squashfs
sudo mount live/filesystem.squashfs /mnt/tails-squashfs
Now you have to copy the contents to a new folder, put your encrypted file into it, and repackage the squashfs file.
Now all you have to do is repackage the ISO, and you're good to go.
sudo cp -r /mnt/tails-squashfs ~/tails-squashfs-copy
cp /path/to/your/encrypted/file ~/tails-squashfs-copy/home/clearnet
sudo mksquashfs ~/tails-squashfs-copy live/filesystem.squashfs -comp lzo -noappend
Time to clean up.
mkisofs -o tails-modified.iso ~/tails-dir
Also, I highly suggest you don't encrypt your file with OpenOffice. If it works like MS Word, it encrypts the file using the old and very weak RC4 algorighm, which any half-decent computer today can brute force in minutes, REGARDLESS of how long your password is. If you want to encrypt your file securely, you should do it like this:
sudo umount /mnt/tails-squashfs /mnt/tails-iso
sudo rmdir /mnt/*
rm -r tails-squashfs-copy
And to decrypt,
openssl aes-256-cbc -salt -in my_secrets.txt -out my_encrypted_secrets
If you require the level of security Tails provides, then relying on application-specific encryption is very foolish, as it tends to be quite weak (e.g. MS Word, BitTorrent encryption, most SSL, WEP, all of which are very weak and all of which use RC4).
openssl aes-256-cbc -salt -in my_encrypted_secrets -out my_secrets.txt -d