The problem is Peptide nucleic acids (PNA) have different helical parameters (i.e. rise, twist etc.) than RNA. So, if I build a RNA and then modify the backbone to build PNA, it will still have all the helical parameters of a RNA.
Just use the PNA helical parameters to build the RNA. That way, the bases will be placed correctly. Then modify the backbone and minimize with restraint on the bases.
I couldn't find anything to build PNA. Structure files are basically text files. So, you may try to write a script to replace the atom symbols or delete some lines. For example, with a script you can delete all the lines containing information on ring oxygen and the H of the OH group; replace the atom symbol for one backbone O with N and the other with C, so on.