I just got a 7.2V  2000mAh NiMH for my Frsky radio but I am not sure how I should charge it.

Should I set my charger to NiMH then 0.5A and 9v? And then recharge it again once it get close to 7.2v?

I don't know what kind of charger you have, but most charger for lipo will have a NiMH option. For 7.2V NiMH, that is 6 cells. NiMH should be charge at 1C. To do the math, you should be charging at 200mah on 6 cells.

If you don't have NiMH option, I would not risk trying to charge.

best to use a dedicated charger for safety, in your case this:
https://amzn.to/2T7HbeT

Alternatively, if you want the convenience of being able to charge the battery in the QX7 without having to remove it each time like you can with the X9D then you can look at fitting a neat in-transmitter charging kit like the following (or get the relevant connectors and make your own). Be aware that this should only be used to charge Ni-MH batteries inside the QX7 and not LiPos or other types or batteries.

https://alofthobbies.com/qx7-charger-kit.html