You are located in The Netherlands, so you need to determine the local codes for electrical wiring. You need to determine where a junction box can be placed, size of wire requirements, breaker requirements, etc.
Basically, you should have a junction box, another wire of sufficient length that is identical to the light wire and a conduit of same length. Join the 2 wires together in the junction box and have the new wire in an approved conduit from the junction box to the transformer box.
Ensure you keep like colored wires attached to each other to avoid confusion at the connection in the transformer box.