You can't convert a file to a font, a font to another font, or text to another font. Fonts are designed to be installed and used as they are. You can convert text from a language to another language's own text and that's it. There is no tool to convert to Mangal fonts.
Secondly, there is a page with all kind of tools that might help you convert you text to Mangal text format.
Use the following and download the Input Tools for your language: http://bhashaindia.com/Downloads/Pages/home.aspx