I would say that root grafting is an inaccurate term. You will most likely not have the facilities nor the fine motor skills needed to graft roots; in addition your plants or roots would most likely die due to exposure to air/light.

What you're wanting to do is graft a cutting (not clone) onto a plant with an established root system . This means the plant would have to get cut with some stalk remaining. This would shock the root system and it would take considerable amount of time to recover. In addition any root pathogens that existed would effect the plant you grafted to it. Lastly; the roots would still need space to grow. So if you did successfuly get this to work then you would still have to increase the pot size a few times.

Due to the time it would take to recover from shock from cutting down the plant.. shock/acceptance of the grafting and trimming the rootball or potting up to stimulate new growth; you would probably be better to just let your clone root for 1-2 weeks and then put it in your hydro or soil grow.

Not only would this be faster but you would end up with a much healthier plant.

Can you graft a plant onto a plant with an existing root system; yes. Is it worth the time and effort it would take? I would have to go with a solid no but that's just my opinion.

good luck.