This is possible, but will be fiddly.
You'll need to identify the feed to the ignition switch, and the wire that goes out to the starter, or to the ECU to request the starter.
Feed to the separate switch needs to be from the ignition switch, not the feed to it, as the starter switch needs to be live only when the ignition is on.
Then just take the wires to a push switch.
something similar should be available in Europe. Those who race or rally cars will have such things, so there will be a source.
Do you have a lot of keys on the same bunch, all hanging from the ignition switch? This can be a cause of switch wear, with the weight pulling down all the time. Ideally, only ignition key and fob should be there.
Thank you very much for your input. The key bunch weight is something i didn't consider before.
The hints to racing are helpful to.
Concerning the wiring i can say that i'm not the worst one in eleckatricketing. (Gave a worn out Mercedes Diesel OM 615 another 200,000 km by building in a 24V equipment from a lorry after about half a million before as a cab.)
I consider finding out the wires at the fuse box can be a task for a warm, not to sunny Saturday afternoon.
My experiences are a bit outdated, though, and i know that i have to consider that the hardwired computer which is running my van's motor is always on the run to detect a lie which gives it the opportunity to recommend me to the friendly garages around.
Since i know from experience that a 16A relais is good enough to switch the amps for an (old) beetle magnetic starter switch i wonder whether an ordinary 16A switch will do, giving me the opportunity to leave out the relais. (In German cars, i never saw the wire gauge to be a problem. The latter became a problem with the first Italian cars i met, even on an Alfa. This problem is new to me.)
Does anyone have an idea how many amps the solenoid switch of a modern diesel starter will draw when the oil is cold, the battery is longing for being loaded, the sun is four hours away from rising, and the wiring is designed by FIAT?
Anyway, your input is very helpful. Thank you very much for that.
(Yes, i confess that i'm a lazybone. My van has to go whenever i decide to want it to go. It needs some care which i'm willing to give preferably on warm days in an easy-going working environment rather than at four o'clock in the morning with -10 degrees somewhere on a country road with no net on the phone.)