By default, at least on the DG834Gv5, the option to add your own entries is not enabled.
Log into your netgear with telnet.
With a web browser, connect to http://your-modem/setup.cgi?todo=debug
Telnet to your modem: telnet
Create a new hosts file with local addresses:
cp /etc/hosts /var/etc/hosts
echo "192.168.1.10 www.myinternalwebserver.com" >> /var/etc/hosts
To check it works, restart the DNS Masquerade program:
dnsmasq -h -n -c 0 -N -i br0 -r /tmp/resolv.conf -u r -H /var/etc/hosts
Making things permanent:
Now, there is a problem making this permanent as the root filesystem is 100% full, but what you would need to do is:
Mount the root disk read/write: mount / -o rw,remount
Copy the hosts file over: cp /var/etc/hosts /etc/hosts