The Colombian has been released by current team Paul Bird Motorsport so he can make the switch.
Hernandez will take over from Michele Pirro as the stand-in rider on Ben Spies' bike.
Official Ducati test rider Pirro is required by the factory for development test duties in the coming weeks, so Hernandez will join Andrea Iannone in the Pramac line-up until Spies is fit to return.
Hernandez is in his second year in MotoGP but has only raced in the CRT division so far, riding for Avintia in 2012 then moving to PBM for '13.
The 25-year-old will make his Pramac debut in Monday's Misano test.
It is not yet clear when Spies will be fit to return.
Lingering problems from the shoulder injury he sustained last autumn meant Spies has only started two races since joining Pramac.
He took a long break in early summer to try to recover properly, only to crash in practice on his return at Indianapolis and sustain a dislocated shoulder.
- Sports & Recreation
- Ben Spies
- Michele Pirro
- Andrea Iannone