Sending form data to another post?

Here’s what I want to do:

On PostA, I want to have a handful of text input fields. The visitor fills in these fields, then clicks Submit. That data gets sent to PostB, and is used to populate that post or page.

I can do this very easily with a plain html and a plain php document, but I can’t do it in WordPress.

I’ve looked for form plugins, but they all seem to only send the data to email. I need to send it to another post or page.

Any ideas how I can achieve this?

Thanks much