As I understand, you want to convert your MMORPG from Unity to Playcanvas, Unfortunately there is no ‘automatic’ way to do that, you will have to rewrite everything from scratch. @Leonidas made a tutorial to port a simple game from Unity to Playcanvas: Converting a Unity3D project to PlayCanvas - Part 1 | Playing in Canvas.
Yes, it is definitely possible to build multiplayer projects with PlayCanvas! Check out the guide I wrote on Photon Multiplayer with PlayCanvas here. Since Photon has an official Unity client as well, it’ll be a lot easier for you to pick up too.