This is not so easily done. As for gameplay stuff such as attacks, enemies, objects just replace kernel.bin and scene.bin with the psx equivalents. The dialogue is however contained in the Field files which are not identical on psx and pc. On the pc all stuff is located in flevel.lgp and on the psx its in the Field folder as ."bcx" files. Afaik those bcx files have only been edited through hex editing so you wont probably find a smarter and quicker solution than opening the iso in makou reactor to view the psx dialogues and copy em over to flevel.lgp using loveless, scene by scene. Hope to be proven wrong
EDIT : Sorry I meant .bsx files^^