Hehe, nice try.
The whole alignment can be done in KiCAD and relies on the original mechanical data from the foundation.
You just need to know your way around the tools and how the coordinate readout in KiCAD works.
No eyeballing required.
Raspberry-Pi-B-Plus-V1.2-Mechanical-Drawing-1.pdf (42.5 KB)
How do you add a GPS, 4G, Wifi or Bluetooth module to your board?
You add a symbol in the schematic and then have a footprint associated, right?
The only difference between an add-on board and a solder-on module is the type of mounting.
This means, the footprint for the add-on module will actually need to contain the footprint for the interface connector + mounting holes if any (just scroll up to see how this looks like for the RPi boards).
With KiCAD you will run into trouble if your add-on module contains more than one interface connector though (Arduino), if you want the BOM to reflect this (and the automation pos file), as this means 2+ footprints, which KiCAD can’t represent from one symbol in the schematic as of right now. For this you would need as many symbols as there are connectors, with one of them containing the placement information of the other connectors…
For hobby use this would be no problem at all (single symbol & footprint), you just need to keep track of the additional connectors yourself (ordering).
Anyhow, templates work as well, IMHO just with less freedom from my POV and some chance to screw up alignment without recognizing it (unless you check your work meticulously before having it made).