Welcome to the Community! I appreciate you coming here with your inquiry. At this time there is no way to set up default website addresses for your social media accounts in your templates. However, after you link them if you make a copy of that email campaign then the addresses for the social media icons will carry over. The addresses will have to be re-inserted in any brand new campaigns created.
We definitely appreciate your interest in this particular feature and I will make sure to pass the feedback to our developers for review!