Ok for the donwload display you could use code like this:
<xp:this.data>
<xp:dominoDocument var="yourNewDocument"></xp:dominoDocument>
</xp:this.data>
<xp:panel>
<xp:this.data>
<xp:dominoDocument ignoreRequestParam="true" var="download" action="openDocument"
databaseName="otherDatabase" documentId="calculated">
</xp:dominoDocument>
</xp:this.data>
<xp:fileDownload rows="30" id="fileDownload1"
displayLastModified="false" value="#{download.richTextItem}">
</xp:fileDownload>
</xp:panel>
This will add you Document from the other Database as a dominoDocument dataSource wich you can use to bind a <xp:fileDownload> to.
Then you could add something to select the Attachments what you want to copy to your new Document for that i would recoment a repeatcontrol and use it like:
<xp:repeat id="repeat1" rows="30"
value="#{javascript:download.getAttachmentList('richTextItem');}"
var="attachment">
<xp:panel>
<xp:label value="#{javascript:attachment.getName() }" id="label1"></xp:label>
<xp:br></xp:br></xp:panel></xp:repeat>
Insted of a label you could add a checkbox and in the onSave event you can run some code wich copys the selected elements to the new Document (have to look it up in my app).
You can copy them Using NotesDocument.copyAllItems() or saving the attachments to your server disk and add them with ritem.embedObject to your new's documents richTextItem. But i recomend doing the attachment copy in backend in my expirience working with attachments can be a very tricky part.