Can you give a more detailed example of what you want to do?
If I am understanding you correctly then you can have the user select from the child list via your portal and set a global field with the ID. Then from that global field, your grandchild relationship would be keyed from this global to the foreign key in the grandchild table.
So you select from the first portal ( child ) and set a global field with the primary key, pkChildID.
Now your second relationship will be keyed gSelectedID = grandchild::fkChildID.