Tamil support in epub is technically possible as epub requires unicode support. Whether or not Apple or other readers provide fonts with Tamil support is a different question. You can try embedding your own fonts (epub supports that) but not all readers honor embedded fonts.
As for conversion, I'd recommend saving your Word document as HTML (use the "Web Page, Filtered (*.htm, *.html)" format in the Save As dialog) and convert the HTML to epub using
calibre. From there, you can clean up the epub and do other editing using the
Siigil epub editor.
Good luck!