AngularSPAWebAPI | Angular Single Page Application with an ASP.NET | Authentication library

Further to my comment on your original question. Implement an impersonation feature within the implicit/hybrid flow. If a user is determined to be a "super admin" then present them with an additional step after authentication that lets them enter/select the account they wish to impersonate. Once that's done simply establish the session on the identity server as the selected user (and possibly store additional claims denoting that it is an impersonated session and who is doing the impersonation). Any tokens will then be issued as if you were that user and all without having to know the password.

Additionally if you wish to create tokens yourself have a look at the ITokenCreationService provided by IdSrv4. You can inject that into your own controller/service/whatever and use CreateTokenAsync(Token token) to generate a signed JWT with any claims you like.