Wow that line moves fast on Pro difficulty!
OK, so you need to score a try on Pro difficulty before you can attempt a conversion, which can be pretty tricky, even against the weaker teams.
But there is another way - let the CPU opposition score a try under the posts, then pause the game and switch teams! You then get the chance to attempt the conversion. This may take a few tries, as kicking can be tricky - but you can switch back if you missed, and let the CPU score again (I say let them score, they'll probably be scoring a lot anyway
) - after each of their tries, change teams and try again.
Obviously helps if the CPU team you're playing against has a good kicker....don't play against Brazil or someone!