I think this is Possible and for this you need to register ePO 4.6 in ePO 4.5 & then you can use Transfer systems option from systems tree.
I think will be Possible but i'm not Sure. You can test this first in you test environment.
I have actually test it but im experiencing problem on importing the key from 4.6 to 4.5, seems to be that its having issue on 1024 bit and 2048 bit.
as per mcafee support, need to manualy copy the 1024 bit file from the DB directory and thus, i have succeded the import of the 1024 bit from 4.6 to 4.5 but when i tried to register the server it says that "master agent-server must be imported" thou the key already there.