No. I have just created a .Xdefaults file containing that line but it does not make a difference. Meanwhile I have also found this thread here:
https://www.reddit.com/r/NetBSD/comm..._any_other_os/
But I do not quite understand what the answers are saying: That xdm does not "source" the .profile file and why should it do that anyway? Can someone please explain in more detail?