Available as a DIY kit or turn key preassembled CNC control the Centroid CNC controller is perfectly suited to a manual to CNC knee mill retrofit. While it is a fair amount of work and cost to convert a manual machine to CNC with a large part of the effort related to installing ballscrews and axis servo motor mounts if you have the time to do alot of the work your self you can end up with a quite capable machine tool.
An alternative to converting a manual knee mill to CNC is to find a CNC knee mill to start with. these machine already have ballscrews, axis servo motor mounts and often axis motors that can be reused. Machines such as the Bridgeport Boss, V2XT and DX32 series and others like Sharnoa, Wells Index, Shizouka, YCI YCM-40 and others are made for CNC knee mills and can often be had for less than the cost of a good manual knee mill.