in this pipe, there are several lines in Item.Description field, so when i create a RSS, only first line would be included.. How can i combine all these lines, so RSS would include all lines in it's Description.
The problem is more difficult that you give away: that's not "several lines" in your description, but several sub-items instead. Sadly, they are no tool in Pipes to convert feed to string.
That being said, in your case, it's easy to get all your content at once. Simplify your XPath to //table[contains(@class,'imagetable')], and check "emit item as strings". Check the the result: the whole table is retrieved and displayed as HTML. Depending on what you wish for, you can either let it be or format it to text using regexes.