「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.