Irtsgbr

I’m reading this manga and I came across this scene that the character is describing the new member of the school team that

音駒【ねこま】にあとひと匙【さじ】 欲しかった決定力になり得る存在

*Note: 音駒 is the highschool name.

For ひと匙, I’m assuming the character is implying that this new member is one last piece (or thing; factor; component) that the team has sought after. However, after I have done some research online, I could not find any examples of 匙【さじ】 with such usage.
(Most examples I found are related to cooking recipe, which was not surprising because of its original meaning of spoon.)

Therefore, I’m curious if my understanding is correct? If yes, has the term been used colloquially?

Your interpretation is actually right. 匙 is spoon (for cooking and prescription) as well as spoonful, that's of course, to measure the amount of sugar, salt, or any seasoning.

As you said, あとひと匙 is a figure of speech saying "the last (missing) spoonful of flavor" that will, I guess English speakers would say, "spice up" the team. The metaphor is easily understood by Japanese speakers.

Your understanding is actually very good: You don't need me.

「​音駒
ねこま
​にあとひと匙さじ
​ 欲ほしかった決定力けっていりょくになり得える存在そんざい」

obviously describes this new player.

The Nekoma High has been lacking an amount of scoring ability (決定力). By how much? By just a spoonful（ひと匙）. The new 194-cm-tall guy could now be just that missing piece for the team.

The use of 「ひと匙」 is fairly common in referring to a small amount of something that is totally unrelated to cooking.

The super-literal TL of the phrase above would be something like:

"The existence that could potentially be the final spoonful of the scoring ability that one desired for Nekoma."