Again, no they don't.
The Kanji, which creates the Japanese meaning, and the Furigana, which creates the pronunciation, are totally separate things. Getting the double meaning does
not require mangling the Spanish pronunciation.
虚閃 is the Kanji that Kubo chose for the beam attack that all the hollows use. The meaning of these characters is "Hollow (adj.) Flash". A Japanese person reading these characters would probably pronounce it "kosen" or "kyosen".
Kubo decides he wants to give his Hollow related stuff Spanish names, so he can use Furigana to change the pronunciation of the Kanji, while leaving the Kanji itself and the meaning intact. He chose to give it the Furigana セロ, which is pronounced "sero". The Furigana has no relation at all to the Kanji, the Kanji is there to either just look cool or give some meaningful context to the foreign word that readers might not understand.
Another clue that this word isn't Japanese is that the Furigana is written in Katakana. Most normal Japanese words are written using Hiragana. Katakana has only two common uses: emphasis (like
italics or [b]bold/b]) and foreign loan words. Since the Katakana is used every time they talk about Cero, it's clearly not for emphasis. That means it's a foreign word, and thus meaningless in Japanese. The word "sero" is meaningless to your average Japanese person, unless they happen to know Spanish or are a Bleach fan.
Kubo chose to use セロ as the pronunciation for this attack, but he could have just as easily called it ラ ・ クカラチャ (Ra Kukaracha,
La Cucaracha (sp?)) if he wanted, and he wouldn't have had to change the Kanji at all. The pronunciation and the meaning are totally independent of each other.
This is a very common technique that is used by many, if not most, manga writers in Japan, not just Kubo.
The upside to using this technique is the cool double meaning you can get. The downside is that, if the manga is made into an anime, you loose the Kanji that might have gave meaningful context to the word, and are just left with the pronunciation, which may or may not make any sense.
On a different tangent, what kind of idiot Mexican wrote a song about cockroaches? And made it so damn catchy too? GET! OUT! OF! MY! HEAD!!

Must....drown out......that damn song!