Thanks for your answer. I have made an Armory wallet. I have a paper backup with the Root Key. Is the a general key or is it specific for Armory?
Paper Backup with Root Key is Armory Specific. To the best of my knowledge, no other wallet is able to use this root key.
Someone sent bitcoins to it and that transaction went OK. But I can't get the Armory wallet online uitil I have made a node? Is that correct?
Yes, you cannot get Armory wallet online without a full Bitcoin Node (that is also fully synced).
If I make an Electrum wallet, can I import the Armory wallet or the content of it?
You cannot directly import the Armory wallet. You would need to export the private keys from the Armory wallet, and import the private keys into Electrum
Wallet Properties -> Backup This Wallet -> See other backup options -> select "Export Key Lists" -> [click Export Keys List button] -> select "Address String", "Private Key (Plain Base58)", "Included unused (address pool)" and "Omit spaces in key data"
You will get a list of your addresses and matching private keys in WIF format. You can then import the WIF keys into Electrum (or any other wallet)