Using MSAL to obtain tokens for MS Graph without overriding custom app authentication

Question

I'm building a .NET 6 ASP.NET app that has custom authentication. I need to call MS Graph APIs on behalf of my users and I want to use MSAL for that. Unfortunately, all examples include signing in the users to the app via Azure AD. I do not want MSAL to override my custom authentication, I just want to obtain the access/refresh tokens and I will tie them in with my local user principals.

This is the example code I'm looking to modify:

builder.Services.AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)
    .AddMicrosoftIdentityWebApp(builder.Configuration.GetSection("AzureAd"))
    .EnableTokenAcquisitionToCallDownstreamApi(new[] { "User.Read" })
    .AddMicrosoftGraph(builder.Configuration.GetSection("Graph"))
    .AddInMemoryTokenCaches();

I could manually craft all the OAuth 2.0 messages but surely MSAL can do that for me without overriding my app's User.Current? Any tips, examples?