Technically, it's possible to write the "si" sound as スィ, similarly to much more common constructions like ファ and ティ.

Wiktionary lists eight whole words that use it! (The entry for スィ itself, a couple obscure loanwords and proper names, and a couple alternate spellings of words that Japanese people normally pronounce using the "shi" sound.) https://en.wiktionary.org/wiki/Category:Japanese_terms_spell...