I am trying to put in a box heater instead of my baseboard wire is to short can you splice into a 240 line to extend the wire? I don't want it to be a fire haserd and I was at the hardware store and there was a splice kit with a sleve that you shrink around it but no one new if it was safe with a 240 or should I just put in a box for a plug in socket then continue the wire from there to wear I want to put the heater?

Need to use a junction box and cover, with the proper cable connectors.