Here, "if" or "when" would change the meaning of the sentence, so it is wrong to put these words here in this context. It is an example of a set expression in English. Another example is "What I wouldn't give in order to (+verb or other action)"

Hi Yuuich, There's a mistake in the text. It should be "...in my shoes." The words "in by shoe" have no meaning in any context. Does it make sense now? There is no need for any extra words. By the way, do you mind telling us what your source is? I ask this because I helped you yesterday with another sentence you were trying to understand but which was written wrongly. If you're using the same source to improve your English, I am a bit worried on your behalf.

Hey Yuuichi, "If" and "when" clauses are only used in certain situations. "If" is used to show potential, improbability, or obligation. "When" is used to refer to a specific time (either real or unreal). In the example you provided, "There isn't an entomologist in the whole world who wouldn't give all he has to be in [my] shoe[s] today," is referring to an actual state of being (in his shoes). Since it is a real physical state (and not a time), neither "if" nor "when" are necessary. Also, "to be in someone's shoes" is an idiomatic expression meaning that the entomologist feels lucky. He's insinuating that other entomologists should envy him. To be in someone shoes means to be someone. "They wish they were me." I hope this helps :-) お疲れ様でした! Greg