The question reading once seems to be a bit tricky to answer. Why worry when there is the answer to all questions. So, let’s see how gets converted:
Well, there is no specific tool developed that converts to Bootstrap Grid. But, yes you can convert it with the help of coding using Bootstrap. Let’s take an example of Table Structure to understand this question better:
Firstly, design a table structure using HTML simple coding as shown in the link and also a button to convert it https://jsfiddle.net/6ucfws8h/
Lastly, Apply to code for converting them like the same in Bootstrap grid, refer the link for the same https://jsfiddle.net/q48my1ha/180/
According to my suggestion developers only use tables to structure the pages, and we change them to Responsiveness (which answers as Bootstrap Grid). So, its high time for us to review the new HTML and HTML5 structure and get updated with the Bootstrap Coding. Well, for better understanding and deep learning to HTML and Bootstrap you can always refer to the best material with the exercises over the above-attached link.
HTML : https://www.w3schools.com/html/html_basic.asp