Unfortunately you can’t just plugin in a Zigbee-USB Controller into the HS to control ZigBee directly, but you can certainly use a Hue Bridge with the Homeserver, there are some well working Logic blocks. This only works well for controlling lamps, but not Hue sensors (motion sensors/switches).
I would recommend looking at other KNX developers, as Gira is one of the most expensive manufacturers with low to mid functionality. A very well received manufacturer is MDT, they offer incredible functionality with a much lower price tag.
Since KNX is a standard, you can use any of the hundreds of manufacturers and mix and match as you like.
Door locks (Nuki), fingerprint sensors (ekey) etc. can also be integrated into the Gira Homeserver/KNX.
You could indeed use both – Homeserver and Home Assistant, but that would create more points of failure. Home Assistant works with KNX (through a KNX/IP Router or Interface) but i didn’t test that irl so far.
I’m not sure if “the house is not smart” means that there are no KNX sensors so far and everything is controlled via regular switches or if its just not programmed smartly but if you already have all the actuators and switches, i wouldn’t recommend downgrading to a wireless (either ZigBee or WiFi/Shelly) system. One broken 433/866mhz transmitter in your neighborhood thats acting as a jammer and you can’t switch your lights off anymore… Wireless should always just be the fallback solution to a proper bus system.