Hi,
Please check the samples.
In the advanced samples , it has a forum sample.
It allow the user create a topic, and edit a topic.
The sample is very close to your request.
So please check the EditTopic.aspx
You need create an IAttachmentProvider ,
Set the attachmentcontrol.CustomProvider to your custom provider class
and then use attachmentcontrol.AddCustomAttachment to insert the existing items.
Regards , Terry .