We never recommend our clients to modify Sitefinity database directly as this could cause a lot of problems. Using the API is safer and guarantees that the data will be successfully imported and the URLs of the posts will be properly generated. Additionally, the API methods implement a very complex logic and it will not be easy to achieve this through SQL.
Actually, creating the tool should be a few minutes task :). It could be a simple .aspx page under your Sitefinity web site. It will contain a few methods (one for getting the old data and one that iterates through the old data and creates the blogs and their posts using the API) and a button that will call the method for importing the data. And that's it.
Please contact us if you need any further assistance.
the Telerik team