Yeah, but here's the thing- You have to be a "witness" to be called as a witness.
For example, I did not testify in the OJ trial because I did not witness any of the alleged crime.
Hunter Biden is not being called as a witness in the impeachment hearings because as much as Republicans want to change the subject of the trial the fact is he has nothing to do with the alleged crime of Trump using national security in exchange for a public smearing of a perceived political opponent.
If Republicans or the department of Justice really believe a crime was committed by Hunter Biden they are free to open an investigation into that. In the meantime, his relevance to the subjected impeachment hearings is as relevant as my testimony in the OJ trial.
The president, his chief of staff, state dept head, and NSA advisor all have direct knowledge of the events being discussed in the trial and have no constituional grounds to avoid the subpoenas. But they are anyway with the (correct) inference that Democrats wont spend 10 months in court fighting it during an election year.