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