To answer this question we need to highlight what happens when a modifier is used. At the point a modifier is connected to a dashboard object, a copy is made and the copy is added to the dashboard. The copy has no dependence on the original modifier.
This means that if the dashboard is exported and imported into a new work space or into the web dashboard, there is no need to re-import the custom modifiers you have used. However they will not be available in the new work space. If you have imported a custom modifier, used it, exported and then imported that dashboard into a new work space, then you will not have access to that modifier for any new connections you want to make to other dashboard objects. You would need to import the custom modifier again.
How To["How To"]