JetJet13 is right, there only those two ways to destroy coinage as far as is known.
Coinage will accumulate whether you computer is off-line or your wallet.dat resides on a USB stick somewhere.
Below my response on your questions:
If I export the privatekey of an address in my wallet, will the coinage accumulated in the address be destroyed?
No, as above
If I export the privatekeys of some coins in a wallet, and let these coins accumulate coinage. What happens if I later import the privatekeys of these coins to another wallet? Will the coinage in the original wallet be affected?
The privatekey or the wallet doesn’t hold your coinage or coins. The privatekey should be seen as the way to access your coins which actually live in the blockchain, which is basically in the the peer-to-peer cloud with copies on all clients.
So you can create multiple wallets on multiple computers, but you still have access to one single wallet with your coins.
If you have multiple wallets/accounts e.g. 10 coins in one and 15 coins in another, you need to backup them both (either the private key or the whole wallet.dat which holds the encrypted private key)
If the coinage is not destroyed during importing, what happens if I export the private key of some coins then import it to 10 wallets ? Will the coins in in all wallets accumulate coinage? After 30 days, will all wallets start POS minting? I assume once one of the wallet finds POS, other wallet won’t find valid POS blocks for the same coinage. Can someone confirm this assumption? I would still gain advantage in shortening the time to find POS block by 10 times, right?
No matter how many wallets you export to, you only own one account in the blockchain which they would all point to. Coinage is just a calculation by the wallet based on when you started to own your coins and kept them in your wallet and didn’t mint coins. The information for this is all being held in the blockchain and your wallet (private key) gives access and the client makes the calculations for you and displays the transactions, balance etc.
If I cold-store coins, will the time the coins spent in cold-storage generate POS after the coins are re-activated (by importing privatekey) in a wallet?
When you take you coins out of cold-storage and import the private key into a wallet then they are ready to mint based on your coinage in the blockchain
If you compare it with a normal bank account, the private key, would be your username/password, the wallet is the browser and internet page of your bank displaying your balance. The blockchain is the bank’s ledger.
When you make payments you need to provide your public key, which can be compared with the bank asking for a pin, a code from an authenticator or something similar when transferring money.
Minting can be compared with gaining interest. You only gain interest if you don’t touch your money for a certain amount of time (minimum 30 days for Peercoin). Coinage is basically the calculation of the time you don’t touch the coins. After 30 days you can ask the bank to pay out by putting the coins up for stake.
The more coinage you have the better the chance you have to get a payout (minting new coins). Even if you don’t get a payout after 90 days your coinage is still accumulating (as far as I understand). Only the chance that you can actually mint coins doesn’t increase any further after 90 days.
Hope it makes sense.
Edit: formatting it nicely : and added minting/coinage examples to complete the picture