Just as the title says, would it be correct to use “它” in a sentence like this "它在外面下雪了." or I could just say "在外面下雪了"？
The "it" in "it is snowing" does not refer to anything. It is there because a subject is needed in a declarative sentence in English. Chinese, however, is different in that subjectless declarative sentences are perfectly fine. When we try to translate a sentence such as "it is snowing", it's best to simply ignore the "it". You won't lose anything semantically.
It is snowing. 下雪了.
It is snowing outside. 外面下雪了.
In English, 'it' refers to 'weather' in "it's snowing outside."
In Chinese, we interpret this 'it' as '天', since 天 could refer to 天气（weather） in this setting.
(外面)天正在下雨 // it's raining outside
(外面)天正在下雪 // it's snowing outside
(外面)天正在刮风 // it's windy outside
今天，天（气/儿）很好！ // today, the whether is good!
However, 天 could be omitted most of time. They will become:
外面正在下雨 // it's raining outside
外面正在下雪 // it's snowing outside
外面正在刮风 // it's windy outside
今天，天（气/儿）很好！ // in this case, 天 can not be omitted. Otherwise, there is no clue for what subject is being talked.
Your issue with 它在外面下雪了 is:
you should replace 它 with 天。
外面 is used as adv or adj, so 外面 itself is ok. No need 在.
the sentence order; you could consider 外面 as 外面(的)天， so 天(S) + 下雪(V)+ 了(C) . 外面（的） serves as an adj. to modify 天. So, either 外面(的)天下雪了 or 外面下雪了 works.