Here is a somewhat easy custom to do, puting magnets in a Microman or Micronaut figure. I purchased a collection of "Rare Earth" (Neodymium) magnets from Ebay. I searched around for something small that would work.
I measured the magnets against the feet first then I took a small drill and drilled opening for the magnet. Be very careful not to go all the way through the foot. I then took an xacto knife and cleaned out the hole, and in some cases trimmed some extra material out of the hole for the magnet. I tested my magnets first to see if they would fit in the completed hole.
Finally, I then placed a drop of superglue on each foot and inserted the magnets. I let the glue dry for a day, then my Microman was ready to use.