See the documentation on how to append a tag:
soup = BeautifulSoup("<b></b>")
original_tag = soup.b
new_tag = soup.new_tag("a", href="http://www.example.com")
original_tag.append(new_tag)
original_tag
# <b><a href="http://www.example.com"></a></b>
new_tag.string = "Link text."
original_tag
# <b><a href="http://www.example.com">Link text.</a></b>