Yes, you most definitely can, but first, you need to sync them in Integrations.
Custom conversions are linked to specific ad accounts so if you try to use the same custom conversion for another ad account you need to make sure it is saved properly on the corresponding ad account.
To sync custom conversions go to the FB Ads Integration page, hit Sync conversions at the bottom here:
Done! If something works not the way you expected, don't hesitate to contact us on Intercom or email firstname.lastname@example.org.