This is how I would do it:
Remove the ammount of insulation you need at the end of the wire and use the ends of your fingers to twist the wires tightly together. If this will fit inside the plug, then slide it in the hole, and then heat the side of the plug while applying solder to the wires.
If the wire won't fit inside the plug, tin the end of the wire you want to use, and then use a file to grind the circumference down a little. When it fits, repeat the process above.
Bryan