AutoDMG requires that things be in a PKG format to be installed pre imaging, so you can’t place the files next to the PKG before install because it won’t have that context when auto-installed with AutoDMG.
This means you must place the actionsite.afxm file in place yourself using a separate PKG before the BigFix client is installed, and do the same with the com.bigfix.BESAgent.plist because you can’t use the clientsettings.cfg option in this case.
Also, you can’t use the existing BES Client PKG as is due to permissions issues. You have to alter it to fix it: Poorly packaged OS X BESAgent installer - RFE Please Vote!