My recipe states that I need to bake my fish lasagne for 40 min at 180 degrees Celsius. I will go out and want the food to be ready when I return but will not be back within 40 minutes. Is there a way how I can extend the cooking time at lower temp without drying the lasagne out?
Yes. Use a thermometer. Your lasagna will be done at an internal temperature of 165F (74C). Just lower the baking temp appropriately. I can't know the time and oven temperature that will actually work for you, because you didn't present them, but I hope this helps.
Most modern ovens retain heat very well. I know mine takes more than half an hour to cool from a normal cooking temperature to a temperature which is still hot enough to cook at, just more slowly. If yours has a timer to switch it off you should be able to set the normal temperature or just a touch lower, preheat, and have it cut off 1/2 to 2/3 of the way through the stated time. Then residual heat in the oven will do the rest but more slowly.
However, the first time you try this I would suggest being in the house, to look through the glass of the oven door in case it's doing too quickly, and to test when you think it should be done. A (meat probe) thermometer is a good idea at this stage. On ovens with an analogue dial to set the temperature, you can get a rough idea of the air temperature by seeing where the dial clicks. 120C will cook slowly but not brown, for keeping hot you don't want to go much below 100 given the margin for error, though it will still dry out. Don't forget that opening the door when cooking in residual heat lets a lot of heat out, so only do it when you think it's cooked.
You have not said how much more time you needed or how big a dish you are using or whether you are cooking from frozen or room temperature.
Without such important information, I can only guess. If you are looking for a 5-10 min extension, you may be able to lower your temperature to 155-160C. Anything longer than that, your lasagne could come out different if you were to cut temperature further.
You might want to use a delayed start function if your oven has one, but I would probably not want to delay for a hour or more (assumed frozen) for food hygiene reasons.
Alternatively, you can cook it earlier so that it finishes cooking before you go out, keep it covered inside the oven with its temperature reduced to 50-70C to keep it warm while you are out.