I cannot just make a transaction because I cannot go online with Core.
This is a problem. Then you will have to import them or sweep them, but .. please continue reading.
I want to keep old address on new wallet.
I am going to buy hardware wallet in future!
As I said, if you import only private keys, you will not have a HD wallet in Electrum. So it won't be based on a seed, it won't make new addresses for you when you need them. And this is suboptimal at best.
If there are many addresses in Core and you don't mind consolidating your inputs, import all those in a new Electrum wallet so you can transact, then make a new proper Electrum wallet (HD) and send there.
If the addresses are few and you don't want to consolidate, then, as said, use sweep.
Or, if you don't plan to make transactions in the near future, you just import all the private keys into an Electrum wallet and wait for your HW where you'll create a new seed anyway; and then just send to HW.
As you can see, sooner or later you will have to go away for good from the old addresses. You can keep the wallet.dat file (it's advised to never remove old keys since you never know what you've missed), but you'll have to leave the past behind.