Sure. Add an <input type="text" /> or <textarea></textarea> to the <FileItemTemplate> template. SlickUpload replicates this template in the FileList for each file that is selected. Give the <input> an id and name, and you can use this to access it server side.

Something like:

<FileItemTemplate>                          
    <kw:FileListElement Element="FileName" runat="server"/>
    &ndash;
    <kw:FileListElement Element="FileSize" runat="server">(calculating)</kw:FileListElement>
    <kw:FileListRemoveCommand runat="server" href="javascript:;">[x]</kw:FileListRemoveCommand>
    <kw:FileListElement Element="ValidationMessage" runat="server" style="color:#f00"/>
    <p>Description: <input type="text" id="description" name="description" /></p>
</FileItemTemplate>

Server side, you can access this value as you're looping through the UploadedFiles collection, like this:

        foreach (UploadedFile file in e.UploadedFiles)
        {
            string description = file.Data["description"];
 
            // TODO: do something with description
        }