There is a product called Siemens Conversion Studio that is offered by Siemens. Here are links to the Siemens webpage that describes the product and process. It only works for ladder logic. If you have function blocks, structured text, etc., this tool will not work.
So, how good is the tool? When I have used it in the past, it works well for any ladder element, rung construction, and documentation that is considered part of the standard Rockwell offering. Custom UDT's, add-on instructions, etc., are not going to necessarily be converted without some difficulty. But I would say that it is 85+% efficient and will save a lot of time if you are converting from a Rockwell platform to a Siemens platform.