Can I port existing apps to Solid?

I don’t know the former, but you can’t ‘port’ WordPress. Well you can, but I don’t think that’s what you mean by port (which really means rewrite anything needed to achieve the same functionality).

That would be very difficult because you can’t run PHP (or any of your own server side code) on a Solid pod.

You can still create a blog app (see Solid Plume) and pretty much any other kind of app, but anything that uses a lot of server side code world be more of a rewrite.