I have hardware wallet now. Do i need to stop using offline wallet?

[odolvlobo]

Keep in mind that you can use your Ledger wallet with Electrum: https://ledger.groovehq.com/knowledge_base/topics/how-to-setup-electrum-nano-slash-nano-s

Are this function same as "Import a Ledger HD-Account" in MyCellium Add New Account menu?

I think "Import a Ledger HD-Account" is how you use a Ledger with Mycelium. You can use Trezor and KeepKey, too.

[bitgov]

I just purchased ledger hardware wallet. I'm very exited to use it and transferred all my btc to my ledger wallet.
As i know, hardware wallet is more secure than online wallet. I will transfer to online wallet if need to trade (exchange) only.

But how about offline wallet such as mycellium and coinomi? Do i need to separate my btc to offline wallet also? If yes, why?

You do not have to stop using anything, but for what to share and tire when you now have a better solution of your funds.