In order to send SMS and emails to your contacts, you need your contact's emails and phones to be stored in a corresponding System Field. If you already have that data stored in a Custom User Field, you can easily perform an export.
How to export data from Custom User Fields to System Fields
1. Go to the Contacts tab and filter contacts with non-empty Custom User Field containing email or phone.
2. Select Bulk actions -> Export Custom to System Field.
3. Select which field will be exported to which System Field. For the cases of failed export, choose which tag to apply to the users with an error.
4. After finishing an import you can check the results in the Contacts tab, filtering contacts by a System Field:
NOTE: Please note that you still need to ask your contacts to opt-in before you can start sending SMS and emails.