The crack is more likely to open up again when you use joint compound on its own. You can on the other hand use plaster of paris mix on its own but you’ll have to move very quickly as you’ll only have a few minutes to get the job done before it sets.
To mix joint compound with plaster of paris, it’s easiest to first mix the water for the plaster of paris with the joint compound. Then over about a minute, while mixing like mad, add the plaster of paris powder to the mix. Then wait a minute or two, until it is solid enough to work with.