# you have to grant write access to root user for modification

sudo -i

# edit this file

/usr/syno/etc/preference/<username>/google_authenticator

# Now you can add as many 8digit random code as you wish, but do not touch the first 4 lines:

<SOMERANDOMCODEFORAUTH>
" DISALLOW_REUSE <randomcode>
" TOTP_AUTH
<randomcode>
<youshouldaddcodeshere>
<youshouldaddcodeshere>
<youshouldaddcodeshere>
...