We don't support this within the profile right now. However, you could add a global variable under Settings > Variables that could be inserted into email templates.

If we do that, it will be the same for all users, right?

We want to have more than one person using the templates and to have some templates that invite prospects to a 30 minute call and others that invite them to a one hours call.

Since each person has different calendar links, it sounds like this is not possible right now?

If you cannot support variables in user profiles then I suggest you create 10 new fields called Attribute01 - Attribute10 and let users put in what they want and document their use in their organization.