Is it allowed to add snippet with custom key for tranlsation in theme?

Question

Is it allowed to add snippets with custom keys for tranlsations in a theme or is it allowed only for plugins in Shopware 6? For example: shopware6/custom/plugins/MyCustomTheme/src/Resources/snippet/en_GB/customtheme.en-GB.json

{
  "customtheme": {
    "footer": {
      "someNewKey": "Some value"
    }
  }
}