A real shimmer FX is done by taking the WET reverb signal, route it into a pitch shifter (1oct up)... then delay this pitch shifted reverb for some time (to make it gradually introduce itself), and then finally feed this delayed, pitch shifted wet reverb signal, back into the reverb input.
The pitch shifted and delayed signal should NOT be mixed into the main mix, it is only meant to be routed back into the reverb input... with different send levels back into the reverb, the effect will vary in strength.
It does require some tweaking to set it up, if you do not have a dedicated shimmer FX though... a good software plug for the Shimmer FX is Valhalla Shimmer... then you do not need to think about all the routing requirements.
It's impossible to create this using the FX engine in DSI products... not enough FX slots, and routing options at all, it is not even possible doing it with either MX400 or even PCM92... the key is that you need to be able to pitch shift and delay the reverberated signal "off line" (these processes should not get to the main out), and then route it back into the reverb, and this routing is not obtainable in many FX units on their own... they simply do not allow the feedback routing option, and the parallel processing of the wet signal after the reverb.
It can be done using mixer channels with pre AUX sends:
connect the stereo output of your main reverb unit to a single stereo input channel on your mixer.
send a portion of the returned wet reverb via AUX send into another FX processor that can pitch shift the wet reverb and delay it.
connect the wet signal of this FX unit to yet another stereo channel on your mixer, set this channel to be pre fader send, and turn it's volume all the way down so that it does not enter the main mix.
now send a bit of this channel back into the reverb input by using the channels AUX send to the reverb.
That's basically it... but be careful with the controls... there are many possibilities that will lead to ear shattering feedback loops