The conversion kit "shouldn't" change the dish of the rear wheel. The rim should be centered between the locknuts (aka the ends of the axle which contact the frame).
If you don't have a dish tool, take the wheel to a LBS and have them check the dish. This should only take a few minutes and most shops will do this for nominal charge.
Should the wheel needs to be re-dished, there should be ample margin with the factory spokes to move the rim. When I've had to adjust the dish of rims which were off by a couple mm it simply was a turn or two difference to the spokes on each side (i.e. loosen one side by a turn and tighten the other side a turn).
Only if the conversion kit significantly changed the dish of the wheel would you need to change any spoke lengths. Hope makes excellent stuff so if there is a major change to the dish, I would contact Hope for technical assistance as there likely is another problem (wrong kit, missing part, etc etc).