Yes, that's possible. However, due to the way ACT works, some things need to be taken into consideration.
First, for each parameter you want to control via ACT, host automation must be enabled in Omni (right-click the control, "enable host automation") - ACT only "sees" automatable parameters. It might be a good idea to start with a clean slate in Omni: right-click, "MIDI-Learn" > "Unlearn All".
Enabled parameters will appear in ACT in the order you activate them. So, first all 4 rotaries banks are filled up, then the fader banks. I'd recommend to plan a bit beforehand which parameters you want to control with which knobs/faders of the MC. You can of course re-order the assignments with ACT-Learn, though I believe the less re-ordering the better.
It's important that all assignments you make, will be stored in ACT based on the order of enabled parameters, not their name. So if you enable other parameters in another instance of Omnisphere, that new list of parameters will determine the connections between your MC controls and Omni controls - which is good, because you can have different sets of parameters for each instance. But: if you now perform ACT-Learn again, it will mess up your earlier (ACT-Learn) assignments, as ACT holds only one set of parameters for each plug-in - not for each instance.
Another advice of caution: don't click the automation Write button on Omni in Sonar. I did it once and thought Sonar had crashed - but it was just busy with write-enabling Omni's over 14,000 possible parameters - that took a while. Better right-click and enable only those you need.
Post back if you have further questions about ACT.