Generate access token with IdentityServer4 without password

[HttpPost("loginas/{id}")]
[Authorize(Roles = "admin")]
public async Task<IActionResult> LoginAs(int id, [FromServices] ITokenService TS, 
    [FromServices] IUserClaimsPrincipalFactory<ApplicationUser> principalFactory,
    [FromServices] IdentityServerOptions options)
{
    var Request = new TokenCreationRequest();                        
    var User = await userManager.FindByIdAsync(id.ToString());
    var IdentityPricipal = await principalFactory.CreateAsync(User);
    var IdServerPrincipal = IdentityServerPrincipal.Create(User.Id.ToString(), User.UserName, IdentityPricipal.Claims.ToArray());

    Request.Subject = IdServerPrincipal;
    Request.IncludeAllIdentityClaims = true;
    Request.ValidatedRequest = new ValidatedRequest();
    Request.ValidatedRequest.Subject = Request.Subject;
    Request.ValidatedRequest.SetClient(Config.GetClients().First());
    Request.Resources = new Resources(Config.GetIdentityResources(), Config.GetApiResources());
    Request.ValidatedRequest.Options = options;
    Request.ValidatedRequest.ClientClaims = IdServerPrincipal.Claims.ToArray();

    var Token = await TS.CreateAccessTokenAsync(Request);
    Token.Issuer = "http://" + HttpContext.Request.Host.Value;

    var TokenValue = await TS.CreateSecurityTokenAsync(Token);
    return Ok(TokenValue);
}

For a newly released IdentityServer 2.0.0 the code needs some modifications:

[HttpPost("loginas/{id}")]
[Authorize(Roles = "admin")]
public async Task<IActionResult> LoginAs(int id, [FromServices] ITokenService TS, 
    [FromServices] IUserClaimsPrincipalFactory<ApplicationUser> principalFactory, 
    [FromServices] IdentityServerOptions options)
{
    var Request = new TokenCreationRequest();
    var User = await userManager.FindByIdAsync(id.ToString());
    var IdentityPricipal = await principalFactory.CreateAsync(User);
    var IdentityUser = new IdentityServerUser(User.Id.ToString());
    IdentityUser.AdditionalClaims = IdentityPricipal.Claims.ToArray();
    IdentityUser.DisplayName = User.UserName;
    IdentityUser.AuthenticationTime = System.DateTime.UtcNow;
    IdentityUser.IdentityProvider = IdentityServerConstants.LocalIdentityProvider;
    Request.Subject = IdentityUser.CreatePrincipal();
    Request.IncludeAllIdentityClaims = true;
    Request.ValidatedRequest = new ValidatedRequest();
    Request.ValidatedRequest.Subject = Request.Subject;
    Request.ValidatedRequest.SetClient(Config.GetClients().First());
    Request.Resources = new Resources(Config.GetIdentityResources(), Config.GetApiResources());
    Request.ValidatedRequest.Options = options;
    Request.ValidatedRequest.ClientClaims = IdentityUser.AdditionalClaims;
    var Token = await TS.CreateAccessTokenAsync(Request);
    Token.Issuer = HttpContext.Request.Scheme + "://" + HttpContext.Request.Host.Value;
    var TokenValue = await TS.CreateSecurityTokenAsync(Token);
    return Ok(TokenValue);
}

Use this:
http://docs.identityserver.io/en/latest/topics/tools.html

Use this tool that come with identity server:
Declare it in the constructor, to receive by dependecy injection.
IdentityServer4.IdentityServerTools _identityServerTools

    var issuer = "http://" + httpRequest.Host.Value;  
    var token = await _identityServerTools.IssueJwtAsync(  
        30000,  
        issuer,  
        new System.Security.Claims.Claim[1]   
        {  
            new System.Security.Claims.Claim("cpf", cpf)  
        }  
    );