MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
This could do the mixing in easier way. You need a printer with XY moving printbed and Z-axis capable for high load.https://www.thingiverse.com/thing:3345012
At ebay there is iron powder and Fe2O3 powder with fine quality. I feel, mixing in this extruder will be possible but not be a good result. You have to look for metal-filled pellets or masterbatches. This exists for mass-production but it may be hard to buy 1kg. Ask for samples. Making this running would become a research work.
Filaments with metal-filling are available.