Currently Rosco is only ably to directly install from a debian, yum, or chocolatey repository—so if you have files to install in some other repository/format, you’d need to do some more custom configuration in order to get this to work.
Behind the scenes, Rosco is just using Packer; the default template it uses (for GCE, for example) is here (with other cloud providers in the same directory). By default this template just calls a script
install_packages.sh which grabs any specified packages from the desired repository—but you can configure Rosco to use any custom packer template. So you could write your own template that does whatever configuring you need on an image, and then instruct the Bake stage to use that particular template.
An alternative you might consider as well—you could just (outside of Spinnaker) make a base image containing the third-party software packages and binaries you need. Then Spinnaker could just start from this base image and install the package being build as part of the pipeline, as you can specify a base image to the Bake stage if desired. This would require maintaining the base image—but often these base images containing third-party software change much less frequently than the software being deployed so that might be a viable solution.