What's the use to append a に after みたい?

Question

The みたい and に I'm referring to here are from a song called Don't you see! The first two lines of the lyrics are:

友達に手紙を書くときみたいに
スラスラ言葉が出てくればいいのに

If I understand it correctly, the に is used to link the two lines. But I'm not sure what kind of grammar it is. Although there is a grammar that a な-adjective can be converted to an adverb by appending a に to it, the みたい doesn't seem to be a な-adjective.

P.S. Here is the shorter version of the song, if by any chance it can help interpreting the meaning: link.

Answer 1

noun + みたい works and conjugates like ordinary na-adjectives, and this に is there to make it work as an adverb. The "友達に手紙を書くときみたいに" part as a whole functions as an adverbial phrase which modifies 出てくる in the second line.

http://www.guidetojapanese.org/similar.html#part3

Using 「みたい」 to say something looks like something else

Attach 「みたい」 to the noun that bears the resemblance. 「みたい」 conjugates like a noun or na-adjective and not an i-adjective.

A monolingual dictionary categorizes みたいだ as an auxiliary verb (助動詞), but that seems to be just because みたいな by itself does not form a standalone na-adjective. Perhaps it doesn't mean noun + みたい can't be treated like a na-adjective.