​Can we add additional fields to cards ?

Can we add additional fields to cards (e.g. numeric field for a time estimate for a task)?

No! That would mess up the beautiful and light UI! If you want a field, use a Note or Checklist. If you want a specific set of fields even with simple validation rules (even with workflows) use SharePoint. Custom lists and connect items to cards. That is the reason why we did build it on top of SharePoint in the first place.