Can I charge my AC rechargeable hearing aid batteries via USB PD with converter?
Something like this or this should work just fine for you (but see the entire lineup, you might like different model more). 

The only problem with 12V power banks is that they tend to be rather on heavy side, which can be a burden while hiking.

I think you should also explore solar power alternative. While smaller solar panels won't be able to provide 1A charging current they still should be able to charge hearing aid battery. But even bigger panels are often lighter than 12V power banks. And you can hang them on your backpack and charge on the go.

Finally there are combination power bank + solar panel models. These are heavy but versatile. Many of them have built-in LED light and rugged body, which is extremely useful outdoors.

UPDATE

If you want to reuse your existing power bank, there are DC-DC "boost" modules that can convert 5V into 12V. Like this one or this one for example. They are cheap and they work. However keep in mind, that inside your power bank there is DC-DC already, producing 5V from battery voltage. This dual conversion reduces efficiency of the power bank. The bank with 12V output does this conversion only once.

Furthermore, to get 1A at 12V you need to draw at least 2.5A from 5V power bank. You need to make sure yours can provide this much. And you can calculate how long it can do that. For example 8,000 mAh bank can give you only 3 hours charging. If this is enough for your needs then OK.

This is the reason I suggested solar power in the first place. When you have no way to recharge power bank on a long trip you are back to square one. But if you have solar panel you do not need to worry. So, check out combination devices too (one, two). At least with them using converter module makes more sense, despite efficiency loss.