Is there any way of setting a pre/post build command to run globally, with every project, instead of setting per project?

No, but this can be handled by a plugin if you have a good reason for that.

In fact, what I want is just a workaround in a limitation of FD.
I have a processes killer in my pre-build command, to kill the debugger player before starting another one.

If the FD itself kills the processes when you stop debugging, the need for this feature/plugin will vanish completely for me, although I think it could server well someone else...