Summary
This is a set of scalable/changeable parts that can be used to convert a normal desktop pedal machine into a handcycle pedal machine.

Note you may have to edit the design itself to fit your pedals. What I did was to go buy a set of cheap pedals with the correct thread-pattern to fit into my machine. I cut some of the plastic excess off the new pedals, and then model it such that it can be tied in with cable-ties.

There is a wrist-collar and 2 extenders, one flat and one at 15 degrees. And LH and RH handle (2 versions, one rounded insert, one square). You need to print 2 wrist-collars and 2 extenders and one each of the applicable LH RH handles. You will have to scale it to your handsize. I print in PLA with 30-50% infill to make it nice and solid. Eventually I will print in ABS.

These parts are still being proven and Im making constant changes. So put a watch on it, or check back over time.

2 versions of the handles... one for a rounded pedal and one for a square one. I will be uploading a generic one too (although the square one is pretty generic).

Assembly/Use:
If you have finger mobility you may not need the collar and extender, and can just grip the handle. If you need wrist support, then also use the Collar to allow for 'pull' grip while cycling.
You may need to wrap some tape or cushioning around the collar and even the handle.
The parts can be glued/screwed together once the correct config is found. For this purpose you may need to drill additional holes in the parts in the exact spot it must be fixed.

Pictures attached of a right-hand assembly including the collar. The collar must be scaled such that the wrist can fit into the gap horizontally, but cant slip out once the arm is 'upright'. Some scaling/cutting/sanding may be required...

If you already have a handcycle with para-grips, you can still use the collar and extender to add wrist-support as long as the extender can be fixed to the bottom of the grips that are on your bike.

Note that getting this right is not just a print and go... you will have to try a few things, scale the models. You may also require to cut/tinker with the pedals to get them working.