Is it permissible to add another dish to the wedding feast (walima), and is it considered forbidden, and would I be sinful for issuing a fatwa about it without knowledge?

It is permissible for someone invited to a feast to eat from it as much as he wishes without restriction, unless the host has specified a plate for each person and prohibited adding more. One must be cautious about rushing to issue Sharia rulings without evidence. Allah the Almighty has warned against speaking about Him without knowledge, considering it among the greatest of major sins. Whoever does so must repent to Allah with sincere repentance.