Add availability field

In my project I need to add an availability field with params
```xml
<availability>
  <isAvailable/>
  <daysFrom/>
  <daysTo/>
</availability>
```
(https://yandex.ru/support/autoru-legal/price-list-parts.html)
I didn't find solution for this. It would be perfect to add some custom blocks of xml inside every offer while generating file