I often use Texpad on documents that use the microtype package which makes texpad fall back to PDFLaTeX. Instead I want Texpad to ignore the included package. Here is a way to do this, by adding a fake empty local microtype package.
Make a folder with a fake microtype package:
mkdir .fakemicrotype
echo "\ProvidesPackage{microtype}" > .fakemicrotype/microtype.sty
Add the new folder to the TexpadTex search path: (Command+Shift+. to show hidden files in the folder picker)
Then just use \usepackage{microtype}
. Texpad will still switch to PDFLaTeX but you can override this in the type setting options.