Most apps reading records will validate a record against the schema for that type. e.g. there's nothing stopping you from making a app.bsky.feed.post record with more than 300 graphemes in the "text" field, but that post won't appear in the "official" app/website because it fails schema validation.

Similarly, there's nothing stopping you from adding another field in your post. It'll just get ignored because the app you're using doesn't know about it. e.g. posts bridged from mastodon by bridgy have an extra field containing the full original post's text, which you can display in your app if desired. reddwarf.app does this with these posts.

Here's the schema for bluesky posts, if you're interested: https://pdsls.dev/at://did:plc:4v4y5r3lwsbtmsxhile2ljac/com....