If it starts with a "1" and you have the private keys you can definitely sign a message using that address... it is probably just that Bitpay and CoPay don't provide the functionality within their wallets.
You would need to use another wallet like Electrum (https://electrum.org/#download) and import your private key and use the inbuilt tool for signing messages... Apparently b.info also offers this for imported keys, but I don't like b.info
Or you'd need to use an external tool like "brainwalletx" (http://brainwalletx.github.io/#sign)... NOTE: I DON'T vouch for the safety of using "brainwalletx" and I certainly wouldn't use their website to input a private key... At the very least you should create an "offline copy" of it (download .ZIP and run locally while OFFLINE etc).
You would need to use another wallet like Electrum (https://electrum.org/#download) and import your private key and use the inbuilt tool for signing messages... Apparently b.info also offers this for imported keys, but I don't like b.info
Or you'd need to use an external tool like "brainwalletx" (http://brainwalletx.github.io/#sign)... NOTE: I DON'T vouch for the safety of using "brainwalletx" and I certainly wouldn't use their website to input a private key... At the very least you should create an "offline copy" of it (download .ZIP and run locally while OFFLINE etc).
Importing private keys to latest version of electrum is safe tho but i wouldn't recommend doing the same for brainwalletx nor for B.info even if it is proven to be safe for the last one.
Avoid online sites when you are dealing with sensitive informations.