The API entities exported from Masto.js are just TypeScript annotations which does not include any runtime code. Therefore, if you cast an arbitrary object as a type from Masto.js, you can make TypeScript recognise it as a Mastodon entity:

import { mastodon } from "masto";

let myEntity = {...};
let myEntityAnnotated = myEntity as mastodon.v1.Admin.Account;

However, there are subtle differences between the original data structure returned from Mastodon API and type annotations from Masto.js. Masto.js internally converts a response body from snake_case to camelCase in a module named Serializer:
https://github.com/neet/masto.js/blob/1ea987575960e1bf937999b0d8e062fe453bd0e4/src/adapters/serializers/serializer-native-impl.ts

Currently, we do not export Serializer as a public API, but you can work it around by putting the filename of the module. In this way, you can pass request bodies from the Webhook call to Serializer to get the exact same data structure as the type annotations exported from Masto.js.

import { mastodon } from "masto";
import { SerializerNativeImpl } from "masto/adapters/serializers/serializer-native-impl.js";

const serializer = new SerializerNativeImpl();

serializer.deserialize<mastodon.v1.Admin.Account>("json", `{"snake_cased_property": "value"}`)
// --> {"snakeCasedProperty":"value"}

Please keep in mind that Serializer is a private class at this moment, which means its behaviour may change drastically between minor versions without any announcement.

I think is reasonable to make it public for the Webhook developers. I will consider adding it in the next version.