Not only is it okay, the Twitter button on a website will do this automatically when you are using Japanese locale. For example, I recently pressed the button and it tweeted
Ursula K. Le Guin pressured to make books 'more like Harry Potter' http://blastr.com/2012/07/ursula-k-le-guin-pressure.php @blastrさんから
When replying to someone on Twitter with an @myname, you don't generally add さん to their username because it makes it look a bit like writing a letter, but if you are mentioning them in the midst of a sentence, you will add it unless if it's a good friend, for example, 「.@mynameさんによると・・・」
With other kinds of usernames, さん is completely dependent on context. For example, on 2channel you may see people add it to be polite, or omit it to be casual, or even from time to time use it sarcastically, like 「1さんのお母さんでいらっしゃいますか?息子さんが 2ちゃんにクソスレ立ててらっしゃいますよ。」