"I am not against it."
Translation:אני לא נגד זה.
Technically yes, with some context. למה את נגד השיטה החדשה? - אני לא נגדה. It could work.
You should even be able to say (though I haven't tried it) "אני לא נגד", leaving whatever it is that you are against not mentioned.
Weird that it's not את זה. I guess I need to learn more when it is used and not.
The word את goes with a direct object but doesn't go with an indirect object.
I ate this: אני אכלתי את זה
I sat on this: אני ישבתי על זה
I am against this: אני נגד זה
Note that in this last example the verb is "to be" in English and implicit in Hebrew. The words against and נגד are prepositions, making the object (this) an indirect object rather than a direct object.
They are an alternate form for "אני לא" in the present. This form is preferred by Hebrew teachers, but you can safely use "אני לא" and be understood.