Add Multiple Projects For a Variable in an Action

I would like to be able to add a multiple projects selector for an action variable.  Ex: I have a custom action that copies an existing expense sheet and asks the user to select a project.  Sometimes there are multiple projects the expense sheet needs to go to.  

I can add a custom action to projects and select multiple projects first - but, it isn't near as intuitive and users won't use it.

Thank you.