soccer style is functioning as an adverb modifying
In English, it is fairly common to use a noun as an adjective to modify another noun. That's what's happening with
soccer: it's being used as an adjective to modify
style together with a modifier as a modifier for something else is a fairly common idiom in English. So, the phrase means "kick-start a party in the style of soccer". Another example is "a hot dog prepared Mexican style". There,
Mexican style is a compound adverb modifying
prepared. Sometimes criminals are said to "kill someone execution style", that is, as if performing an execution. A recently famous example of this idiom is Gangnam Style. This use of the word
style fulfills a role similar to that of the French preposition
à; English lacks a preposition with that meaning.
You can look up "attributive noun", "compound adjective", and "compound adverb" if you're curious to get more information.