As the saying goes, there are multiple ways of accomplishing what you want to achieve.
The way I would suggest would be to use (say) FX1 and install the SMR effect into it.
On the assumption that you are currently feeding your individual drums into input channels and mixing them down to the MAIN L/R outputs, you can also route your individual drum input channels to the FX1 bus (at the same time as MAIN L/R).
The FX1 bus forms a separate input mix into effects unit 1. I use post fade, but you can use pre fade if you wish.
The effects unit has a separate send mute and fader (feeding the input to the effects unit) and a separate return mute and fader (feeding the output from the effects unit).
You would route the FX1 output (effect return) to the MAIN L/R mix.
I usually set my FX1 return fader at 0 dB.
If you press the blue FX1 button on the surface, the faders should ‘switch’ from the ‘main mix’ to the ‘effects mix’.
You can now activate which input channels to feed to the effect (by pressing (and holding) the assign button and pressing the appropriate green Sel button - so the LED illuminates). You can now release the assign button and set the fader levels feeding the effect. The ‘main fader’ has now changed its function to ‘effect 1 send’.
You now have the option of a ‘dry’ drum mix to the mains, and you can ‘add’ a reverb into the mix of a selected set of drums with different levels to those of the dry mix.
I am sure I have missed a ton of configuration steps out, so ask if you fancy attempting this option.
One thing you will need is to have control of the FX1 return fader somewhere (on one of the surface layers) and you can also select the tapping point on the input channels where you want to take the effects feed from. This can be different to the dry mix.
You should be able to see the effect units (and the configurable signal paths) on the block diagram. Finding some of the menus to accomplish the desired configuration is the challenge…
Dave