I guess, I found a nice way to add ofxAddons to Ofelia (besides updating it with the current libs).
Thats the current state:

• Implemented internal Addons are:

ofxAssimpModelLoader
ofxOpenCv
ofxSvg
ofxVectorGraphics

• Implemented external Addons are:

ofxImGui
ofxVolumetrics
ofxStableDiffusion
srtparser

https://github.com/Jonathhhan/ofxOfeliaExtended

Only tested the Windows build for now, but to build it for Linux should be much easier (no idea regarding OSX).
I can upload the Windows build somewhere.

And I opened an issue here, if someone wants to join:
https://github.com/cuinjune/Ofelia/issues/91

ofxSvg needs a small edit, because ofXml::find does not work with SWIG: https://github.com/Jonathhhan/openFrameworks/tree/ofxSvg-SWIG

I made some progress, multithreading works now. It would be nice, to enable multiviewport support. And somehow it only works (at least with Windows) with an old libpd version (the one from the last Ofelia version). Would be great to update this, because this version crashes if audio is enabled while a Ofelia window exists.

Are there any build instructions for the Pd external? In the GitHub link I can only see instructions on how to build the OF addon.