How to verify X509 cert without importing root cert?

My program contains 2 root certs I know and trust. I have to verify certs of trustcenters and "user" certs issued by the trustcenters which all originate from these 2 root certs.

I use X509Chain class to verify but that only works if the root cert is in the windows certificate store.

I'm looking for a way to verify the certs without importing theeses root certs - somehow tell the X509Chain class that I do trust this root certs and it should check just the certs in the chain and nothing else.

Actual code:

        X509Chain chain = new X509Chain();
        chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
        chain.ChainPolicy.ExtraStore.Add(root); // i do trust this
        chain.ChainPolicy.ExtraStore.Add(trust);
        chain.Build(cert);

Edit: It's a .NET 2.0 Winforms application.