No, that'll never happen. Why? Because offline characters are stored locally, meaning that you can edit your character manually. Giving it more stats, levels, gold and whatnot.
If something like that was transfered to online, it could potentinally ruin the whole economy of the game.

lol...what economy?...no AH, no competition, no ladder - NO economy.
And how would a dude with more stats, levels, gold and what not affect you, your char, your game or your life?
What we have right now are basically 2 different games with the same name so we have to spent twice the time for NO reason...

While I agree that it shouldn't "forever" be allowed because the 'falsified' characters should remain offline - It's so early in the release that a lot of char hackers etc aren't rolling out that to make amends for their poor planning they should allow a 'one time' flag to move an offline character to online since people can't play online - Or people who created offline not knowing they weren't sync'd.

They could also make this a long term solution by leveraging Cloud Saves on Steam as a lot have suggested; The fact that it is ONLY stored locally vs cloud, esp considering it's "Multiple OS Compatability" is another stupid over-sight.
Leveraging Cloud Saves gives them the ability to ensure your hash is still natural to what it was 'last it saw it' - Still hackable, but more steps.

That being said, I am okay with offline and online not being interchangable, but they SHOULD make an exception due to their mediocre launch.