It is still not clear what “it” is.
As far as I can reconstruct, the desired feature is an option to change the parsing of floating point literals, such as -2.37. As usual, the devil is in the details.
Changing it globally (based on a compiler option, at startup) would break a lot of code that assumes that those literals are Float64. My guess would be that you would not get past loading Base to get a REPL without an error.
Changing in the context of loading a package or a single file could, theoretically, be achieved, but that seems to be insufficient for your purposes, and is also kind of icky because a global state would affect parsing.
I wonder if we are facing an XY problem: you want to do something, know a feature in Fortran that does it, and you want to do the same in Julia. But, thank God, Julia is not Fortran. You cannot just transplant a feature like that.
It would be great if you posted a self-contained MWE of what you are trying to do, then you could get better advice.