Arcos,
first question is - does the crack detract from the bowl? If no would it hurt to leave it.
Filling a crack, there are several methods, all involve filling it with something. Epoxy Glue (Araldite) is probably the one you have available in your workshop.
You can mix it with the sanding dust to make it blend in,
or, if as you say, the crack can be fairly well closed, mix it with something black, (acrylic paint, or powder poster paint, pretty much anything, I've even used laser printer toner before now). then when it sets, it will look like just another spalting line.
It will over fill and will need refinishing though.