entware allows you to install applications/software that can't be included into tomato itself mainly due to the limited flash size availability. For the sake of this example we'll use a USB pen drive mounted under /mnt/sda1:
First of all create the opt folder and mount it:
mkdir /mnt/sda1/opt mount -o bind /mnt/sda1/opt /opt
You can now run the installation command relevant to your build:
ARM: wget -O - http://pkg.entware.net/binaries/armv7/installer/entware_install.sh | sh MIPS: wget -O - http://pkg.entware.net/binaries/mipsel/installer/installer.sh | sh
At this point you can install packages e.g.
opkg install htop