Hi yaseenhanini,
I think the product won't allow you to re-encrypt again (in a new layer) an already encrypted file.
What you can try is to archive (.zip, .rar, etc) the first .pgp file and then encrypt that archive. In practice you will have a .pgp > .zip/rar > .pgp, but I think it would allow you to reach what you need.
However, I'm curious to know the business case. Can you share?
What is the purpose of reencrypting a file? (To use different keys? The recipient wouldn't be able to decrypt the inner .pgp ...)
Rgs,
dcats