Does the external wallet have a QR code for sending? It contains a private key which looks kind of like: 5HrVwYrQsUcUaKgvvnzyDSR3Q5r65QYF6h9MrFZhkNGpKSg1B6d

If you already have such a key for the wallet, you can put it into the blockchain wallet. On the left under Settings, Addresses, look for Import Address

If you need to scan a QR code to get the key, using blockchain's app would be most secure. Otherwise any QR code reader can read the key, but you would then have to get it securely from your phone to your computer.

Once imported to blockchain wallet, you will see the balance appear. You can then send from that wallet directly to any address.


Remember that it is not strictly an external wallet after this, but the same process would be done with the software client. You can keep using it as long as you trust blockchain.


[edit] By the way the private key I posted is fake. Never post or share a private key with anyone unless you want to give them permanent control of that address. Your blockchain wallet is encrypted and runs in your browser so that your key stays private, so importing the key there is not a big deal.