Hatstand
Without being able to change velocity curves there is probably not a lot of options other than practice, practice, practice.
I was bemoaning my "lack of articulation" with pads and thought it was the pads fault until someone who is a drummer made me look foolish by making a really expressive beat in minutes.
I am now a lot better at it but for three levels of soft medium and hard which is enough most of the time, anything special I just edit velocities in the software.

Practice?  Practice??  What's practice?
 
Seriously, I had thought of that as well, but I'd probably just use my keyboard which will give me fairly good velocity control.
 
My workflow is to separate out the kick/snare into 1 midi track, the hi-hats and cymbals into a second and the toms into a third (all feeding 1 instance of AD2.)  I don't really want to be a "pad drummer" just want to get a humanize beat down.

Assuming your pads are sending 127 on every hit, then there is a limited amount you can do in SONAR or other software to fix this. 127 is a constant number, if you subtract a set number from that constant you will just get a universally lower number. You will not be able to get any new information from the pad. If you apply some sort of randomization, you will just get random results, not the expressive results you are looking for. Any change you make that will do what you want will have to come at the point where the transducer in the pad converts the percussion to MIDI. So if there is not a physical adjustment to the electronics of the transducer, or a software setting in the firmware or driver that will extend the sensitivity of the pad, you will need to reduce the percussive force i.e. hit it softer or put a muffling substance between the drumstick and pad for example using padded sticks.

I'd also either try to play differently or put some kind of damping material on your fingers or the pads. Trying to hit the pads with your fingers more horizontally (as if drumming on a table) might also yield better results than hitting them perpendicularly with the full force of your wrist/arm behind them.

That is why I bought the midi drum set so I could use sticks. It's near impossible to get realistic drum rolls etc with fingers,,, you need the bounce involved. 
All you need is one good drum pad, but the whole kit is awesome if you can at least sort of play real drums.. saves a tonne of time for me.. get most parts in one pass. On a good day, I don't need to fix  much in editing. 
 
My Roland a49 has a slider that when assigned velocity turns the controller from sensitive ( piano ) to hard (organ) 

Sanderxpander
I'd also either try to play differently or put some kind of damping material on your fingers or the pads. Trying to hit the pads with your fingers more horizontally (as if drumming on a table) might also yield better results than hitting them perpendicularly with the full force of your wrist/arm behind them.

I just did a little experiment using my fingers horizontally and bending at the knuckle and the results are much better.  It will take some practice to get a fairly consistent velocity (but not uniform), but this might be the way to go.

did you mention practice?
:)
tbh I found using keyboard keys harder than pads as it is more difficult to hit 127 however my keyboard does have velocity curves so was able to set it to high and get a decent result