That does look a lot like aluminium. All grades of aluminium are non-magnetic as are some stainless steels. The best test is weight as aluminium is significantly less dense than steel but has lower tensile strength so judging by the bulk aluminium seems likely. a final test is that if you grind aluminium with an abrasive wheel it will produce virtually no sparks.
I've not heard of flux core wire for aluminium, stick rods for aluminium alloys do exist but tend not to give great results in general.
Generally TIG is the best option for aluminium alloy welding but a decent MIG machine with pure argon shielding gas and the appropriate filler wire can produce good results. The difficulty with MIG is that aluminium wire is significantly softer than steel so tends to have more problems with wire feed.
Another issue is that not all grades of aluminium have good weldability this article gives good overview.
The other issue, which applies to welding any finished part is that the welding process inevitably puts a significant amount of heat into it which can affect its mechanical properties on a material level an also cause dimensional distortion, which can be mitigated but never entirely eliminated.
The short answer is that it may not be impossible to do but is likely to be considerably more difficult than just welding a bracket to bit of mild steel and modifying finished parts by welding is always a bit risky when you don't know the specific of their material and manufacturing process.