Replace door bell push button

Our doorbell push button needs replacing. I have the new push button, the bell is inside, it is working fine. The electrician that installed this originally didn't leave a lot of slack on the wire, so a wire extension needs to be added, before attaching the door bell itself. I want the extension because otherwise we're dealing with short wires again when the bell goes off again in the future. Also please weatherproof as best you can with silicon sealant.