[Bug] Adding external Parquet data files to an existing Paimon table

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Paimon version

I’m evaluating Apache Paimon for migrating an existing data lake and would like to clarify the intended design and supported patterns around adding external Parquet data files into a Paimon table.

Any solution on it?

### Compute Engine

NA

### Minimal reproduce step

NA

### What doesn't meet your expectations?

NA

### Anything else?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

The report names no files, tests, or entry points and provides no reproduction. Start by clarifying whether external Parquet files are expected to be attached to an existing Paimon table and identify the supported ingestion path. Done means a documented, reproducible supported pattern or a confirmed limitation.

Written by the indexing model from the issue text.