You should use hydro nutes for hempy, and keep the nutrient solution pH in the 5.8-6.2 range (hydro range.)
I have had hempy success using Botanicare Pure Blend Pro hydro nutes, others have had success using other brands...

I get all my nutes from Discount Hydro, they're very reliable and the prices are great.
Here's a link to their site.

Discount Hydroponics



You can try if you want, but I would not recommend attempting to convert a mature soil-grown plant over to a hempy, as you would want to remove as much soil from the roots as possible & this would cause major shock/stress issues.