Is it possible to change a social network login to a normal login for an ArcGIS Online organizational account?

Answer

No, it is not possible to change the login method of an ArcGIS Online organizational account created using social network (Facebook or Google) credentials. This is by design.

To enable a normal login, reassign ownership of all content in the account to a new account in the organization. However, only administrators of the organization have the privileges to do so.

1. Add a new member to the organization without sending an invitation. Refer to ArcGIS Online: Add members without sending invitations.

   Note:
   The option to add members without sending invitations includes the creation of a new ArcGIS Account. This newly created account includes a user name, and a temporary password that is changed during the first login. 

2. Reassign the ownership of the items to the newly added member. Refer to ArcGIS Online: Manage items for a specific member.
3. Revoke all the licenses associated with the previous account, and reassign them to the new member. Refer to ArcGIS Online: Manage licenses.
4. Delete the previous member. Refer to ArcGIS Online: Delete member.

Note:
The account can still be used as a public account. To delete the account from the Esri system, contact Customer Service.