Enlarging the thread diameter can be easily done on a CNC milling machine that's programmable for helical interpolation.
A good shop will probably charge you around $100 plus the cost of the thread mill if they don't already have one that will a 1mm pitch thread. It could also be done with a single point threading tool.
This video shows a CNC router cutting a plastic part. The last portion shows the thread milling operation.
Chas. Colerich
Oakland, CA USA