Do you mean replacing one strand in the DNA double helix with one strand of PNA?
First of all, the helix is not some superstructure to which the molecules making up DNA are attached. Rather, that structure is the result of the shape the molecules take as they bind to each other.
However, in terms of attaching a strand of PNA to DNA, I presume it would be similar to how two DNA strands combine to form the double helix--hydrogen bonding of the bases and generally complementary base pairs (although mismatches are possible).
If I recall correctly, it is the hydrogen bonding that is actually responsible for the double helix shape.
Here's the wiki article:
Peptide nucleic acid - Wikipedia