Power 240 v ac light from two power sources
If I've understood correctly, you want the existing lighting to turn either when the pull switch is activated or when the door opener is activated.

To accomplish this use a relay. The relay's coil would be powered by the door opener and the relay's switch terminals would connect to the terminals of the existing switch. In this way the light will be powered from its existing source whenever the pull switch or the relay (or both) are activated. The relay protects against a variety of bad results that could occur if the door opener output had been connected directly to the pull switch or lighting.

You'll have to choose a relay with appropriate voltage and current specs, and consider local requirements and customs to choose a junction box to contain the relay and wiring. I'd suggest that the relay should be located as near to the pull switch as possible.