The short answer is: yes, you can put a Creative Commons license on your ETD, and this does not impact ProQuest's display of your dissertation.

The longer answer is that within the ETD agreement with ProQuest, you are agreeing to a non-exclusive license  between you and ProQuest. This means that you are granting ProQuest special permission to use your ETD in their service. If you choose to put a Creative Commons license on your dissertation document, that specifies general reuse conditions for your work; however, your agreement with ProQuest specifies the particular reuse conditions between ProQuest and you, which are different from the more generally applicable CC license. 

So by all means, go ahead and use that Creative Commons license. Do you have more questions about which license to choose or what the agreement terms mean? Contact us for more help!