QB Desktop 2018 - Can i add the Class field to the Journal Entry screen?

the title says desktop, the thread is labeled as QBO, which are you asking about?

in desktop if you have class enabled in preferences, then class is available on the journal entry screen

Journal entries should be the exception when using QB, they often do not work as you think they should, and  when you use inventory type items they never work for inventory. It is much better to use the forms on the home page the way QB is designed to be used.    Journal entries also bypass accrual/cash reporting, and will not show on many reports.

Using Class in JE means you risk mixing Class there, as well, along with your attempt to split and allocate the details. That creates out of balance Bal Sheet reporting. By using the "proper" transactions, the Class data stays in balance. Example: Using JE for Allocating income or splitting expense, ignores that the Asset (banking) associated is supposed to stay "by class" for the same entry. The program even warns you in a pop up about Bal Sheet and JE class errors.