Adding support for more wireless dongles would be easy enough as an extension package, it just needs to put the new driver in flash and load it at boot time. The Humax software uses the Linux Wireless Extension interface to configure the driver so the rest should just work.
There just hasn't been a lot of interest so far because compatible dongles are available very cheaply.
I mean the source code of the custom firmware. Yes I no singles are cheap these days but I have some in the house that I don't use and can't use them in the setup box due to not being supported and I don't want to buy more due to the fact I alleeady have some here.
Sometime ago I compiled a driver for a Realtek 8192 based dongle. When the driver was loaded, the Humax wireless menu would become active but it would not find anything when searching for a network. If the SSID details etc were entered manually it would not connect. The dongle did find the network and connect successfully using the wireless-helper package but the Humax app. still would not recognise it.
There is no such thing. The custom firmware is built by taking the official Humax firmware and applying a number of changes to it so that other packages can be hooked into it. It is possible to compare the official and custom firmware by unpacking them with humidify.
The official Humax firmware is closed source but Humax do provide downloads for the source code and configuration they used for the Linux kernel and Busybox environment and a few other components.