Yes, you can add custom fields to your personal settings in Zoho CRM. Please follow the below steps,
Goto Setup > Under Users and Controls, select "Users" > Choose any one user and select the edit icon which will open a pop up window to edit the user information > Select "Manage fields" at the bottom of the pop up window > Add the necessary fields from "New fields" section in the user and address information section and save. Now goto "Personal Settings" to view the field that was added.