SketchUp provides it’s own version of Ruby that you can’t replace or
mix with a another installation.
win32api is a Ruby C-extension and its file extension is “.so” so that
part is correct, you will find it on any installation of
RubyInstaller.
However, if that file is missing from sketchup, you could try placing
in the valid Ruby paths of the tool, but I recommend not to, specially
since things could explode on your face.
AFAIK SketchUp uses an old version of Ruby and RubyInstaller has been
provided for modern versions, mixing them could lead to issues.