Update content Task workflow

[leone-91]

Hi,
I have a "basic" question concerning the update content task in the workflows. I would like to use that task in order to add some content elements in my page (comments of users). I created a form on that page and I would like to publish on that same page the content of the form when the user submit it. However I do not understand how I can update the content without deleting the previous comments as we are supposed to write all the content in the update content task. The "create content task" seems not to be an answer because I want to create content "inside" my page which is already created.
I did not find any specific documentation or demo about this but if you have something, I would be glad if you shared it with me.
Thanks in advance for your help