You can *often* use steering wheel buttons with an aftermarket stereo, but this relies on both finding a head unit with that feature (big names like Alpine and Clarion, rather than Lucky HappyFun £8.59 eBay specials) and an adaptor cable - the head unit will just have an input lead on the back which is specific to that audio manufacturer, then they'll sell you a cable that joins their plug to the cable in your XM that plugs into the back of the factory head unit.
Try googling for "Citroen XM head unit steering wheel adaptor" or something, but don't be surprised if there's not warehouses full to bursting of those - it's would have been quite a "specific" part back in the day, let alone 20 years later.
If you're lucky, Citroen will still be using the same plug for newer models, but it's unlikely.
Edit: Here's an example, from XM to Blaupunkt. There'll be different ones for Alpine, Kenwood etc.
If you really want to use the wheel controls, it might be a case of finding an adaptor then locating a head unit to suit it rather than the other way around!