I have come across sentence and I understand what its trying to say, but I don't understand why に is used with 特別. I was really expecting a の particle instead to modify related noun phrase 旅のサポート
Why wouldn't it be の? Why is it に?
今回は特別に旅のサポートをしてくれる方も来てるんです
My translation: This time, there are even people coming who will provide special trip support!
「今回{こんかい}は特別{とくべつ}（ ）旅{たび}のサポートをしてくれる方{かた}も来{き}てるんです。」
What particles can we correctly use in the blank in the sentence above?
The answer is 「な」 and 「に」. I am afraid that 「の」 is not a natural-sounding choice here.
「特別な」 functions adjectivally to modify nouns only. In this sentence, it can modify either 「旅」 or 「サポート」.
「特別に」 functions adverbially to modify verbs and adjectives. In this particular sentence, it can modify either 「してくれる」 or 「来てる」.
The author/speaker's choice is 「に」, which means that s/he wanted to use 「特別に」 adverbially. S/he wanted to say 「特別にサポートしてくれる」 or 「特別に来てる（んです）」. Which one it is, it is difficult to tell without further context, but I will add that either interpretation would be natural here.
