Not having a Mk1 to look at, but IIRC, adding memory to a Mk1 is a fairly standard SMT solder job - either a pull&replace or an addition to existing free pads. The Mk2/Mk2a would need the new memory to be piggybacked onto the old memory chip(s), and then an address line run direct to the CPU. I doubt that anyone would want to provide that service - too tricky and risky.