I am trying to check if any custom field other that email can be used in email alert action of the workflow . Do anyone knows about it?
Assuming you are talking about Email Alerts, as shown in the picture, the 'additional to' field must be constant values
To provide dynamic email recipients, you either have to:
Recipient Type / Recipientsfield values on the target object of type Email, or a predefined set of related object email values based on the target sobject type (like RelatedContact on Case)
Use Apex Outbound Email Services
If you have text fields on your object that are not of type email, then you'll need to sync them to a non-visible field of type Email using a workflow or process flow Field Update on 'create and every edit'. This will give you a field of type Email available to select from in the Email Alert Recipients picklist.
For lookup fields to other objects, this is more problematic as you have to get the value of the lookup object field into a field of type Email on the target object. You'll need process flows on both the target object (to copy in the value from the lookup object) on create, and, a process flow on the lookup object to copy in the value to all target object children whenever the lookup object changes. Fortunately, process flows allow update of children via Field Update; Workflows do not.
Are you referring to these Email Alert Actions? If so, you could include custom fields in the Email Template your using.
Please try and add more information to the question so everyone has a better understanding of the problem.