Yes, you can do that. It could lead to git conflicts when you upgrade in the future, but it is not too likely.
There is a way to add arbitrary units to the units parser though, via the problem or a macro library. If giganewtons and other things are common for you, perhaps it is worth doing this. See the commentary at the top of:
https://github.com/openwebwork/pg/blob/master/macros/parserNumberWithUnits.pl
for how to add new units. It is something like this:
$newUnits = [
{name => 'GN', conversion => {factor =>10**9, kg=>1, m=>1, s=>-2}},
{name => 'TN', conversion => {factor =>10**12, kg=>1, m=>1, s=>-2}},
];
Then make the answer like:
$answer = NumberWithUnits("3 GN",{newUnit=>$newUnits});
As I said, if you;d use this a lot, then it is worth the effort to not put the $newUnits declaration into each problem. Instead you could make a macro library file that loads parserNumberWithUnits.pl and then sets the $newUnits array ref. I'm not immediately sure how you could write the `{newUnit=>$newUnits}` part into the macro library though. A NumberWithUnits object uses some legacy code and I didn't look into how to work with that.