In this chapter I am going to talk about storing "real" ether, this is a sensitive area: one mistake - and you loose it.
First of all, Wallet and Account are same things. And it is nothing but a pair of Ethereum keys: private and public, stored somehow. Unfortunately, a program used to access Ethereum funds is called a Wallet, too; also a person would expect ot keep his/her keys in a... right, in a wallet. Which is confusing. I'll do my best to use the "wallet" and "account" terms ONLY for key pairs.
Mist (at least at the moment of this writing) can not export private keys, and I was not able to import them regardless the fact that it has a corresponding menu item. It is ok, all Mist does can be done at the lower level, using Geth.
If you already have Ether on an account managed by some other wallet software, you can export a private key from that software (private key is just a string) and import it in Geth using importRawKey command. Use SECOND Geth we opened to enter a command, as it runs in a console mode.
After Geth imported a private key, it should show in Mist as an additional account. You will be able to select it, send money from it and so on.
By default, Mist would store your wallet in "C:\users\john\appdata\roaming\mist\..." folder, which is not a good place to store sensitive files. Unfortunately, block chain files are stored together with wallet data, so if you want to keep them on a removable disk or an encrypted area, you should consider the size of ALL files. However, as we use -syncmode="light" option, the size of files downloaded should not be as huge as it was when entire block chain was downloaded.
I suggest using software like TrueCrypt to encrypt your data and a removable drive to make it easier to transfer. In Setting up Private Block Chain, there is a small appendix explaining how to run a "real" block chain node. Notice the path I use with --datadir command: it is used to overwrite the default location the data is stored.