I want to add anti-freeze X to my system, which already contains an anti-freeze, the source of which I can't remember

Anti-freezes are generally miscible, meaning that if you mix them you get a homogeneous solution, unless they are heat transfer fluids using different glycols (MEG, MPG etc).  However, this does not mean that the anti-freezes are compatible.  Two different anti-freezes will have different formulations in terms of corrosion inhibitors.  

Mixing them together will result in a blend of corrosion inhibitors, the effectiveness of which cannot be determined.  Before adding anything, we recommend that you check the glycolated water present in the circuit to assess whether it might not be better to undertake a complete renewal.