Hi Majid, unfortunately there is no supported way of using workflow to add a number of hours to a date/time value. You know that the integer represents the number of hours, but how does CRM know your integer is not seconds, minutes, days, months or years?
You might be able to use the Date and Time Functions feature on a workflow step to accomplish your requirement, but you're going to have to configure separate workflow rules or conditions for urgent, serious and non-critical cases. You won't be able to use the custom fields you've created on the contract entity as workflow parameters without using a plugin.
Regards,
Neil