Adding Sub Key to PGPPublicKeyRing

[Sandy]

Hi Everybody,
I'm wokring with PGP using bouncycastle API
I'm facing an issue I want to sign a sub key present in a PublicKeyRing
but unable to do so the approach i am following is
1.)Get the Sub Key from PGPPublicKeyRing
2.)Sign the Sub Key using "PGPPublicKey.addCertification"
3.)Now to update PGPPublicKeyRing i used
"PGPPublicKeyRing.insertPublicKey"
4.)Everything seems fine but when i save this PGPKeyRing & try to
import an error comes
the reason is that now both the keys exists in 2 separate key rings
instead of one key ring

Any inputs are welcome

Thanks in advance
Sandeep Rawat