Sure it is possible. You don't need to save attachments to disk before processing. Use MimeText.Text property:
IMail email = new MailBuilder()
.CreateFromEml(imap.GetMessageByUID(uid));
foreach (MimeData mime in email.Attachments)
{
if (mime.ContentType == ContentType.TextXml)
{
MimeText mimeText = mime as MimeText;
XmlDocument doc = new XmlDocument();
doc.LoadXml(mimeText.Text);
}
}