The A20 was originally available with a couple of plug options, including XLR. If you still want this, you need to get a newer A30 as they don't offer it for the A20 variant anymore. There are adapters on the market but not from Bose. You could ask Bose if your headset can be converted to XLR by them, but that means that you cannot use it for GA flying anymore.
Actually, the question of whether you can use the adapter is one you should ask within your company. The rule is in their manual, so it would be best to check with them if they allow it. Depending on the specs of the adapter, it may not meet certain standards in shielding or material use for example.