When defining the value in the input parameters area of a Dropdown (which I can pass to a script action), can I use a property that I have already defined and attached to the blueprint?
Yes, this shouldn't be an issue. In your custom property (CP from now on) which calls the vRO action, simply set the binding value to take from the CP VM.Guest.Datacenter.Network.Pulldown. When you hard-code the value of that property in a property group attached to the blueprint, this value will become the input for the other CP that calls the vRO action. You can then write your JS to return the values you want based on the value of VM.Guest.Datacenter.Network.Pulldown. The downside about this is you have to maintain JS within vRO everytime you want to make a change to these networks.
What's actually far easier is to leverage the SovLabs Property Toolkit to dynamically assign the value of VirtualMachine.Network0.NetworkProfileName based on an evaluation of other properties. So this way, you could have a CP with a drop-down list of static values that contain, for example, "DMZ" and "Protected". The Property Toolkit would then look at your datacenter and environment (lots of ways these can be defined) and then the static value chosen from the drop-down and transparently assign the value of VirtualMachine.Network0.NetworkProfileName based on that unique combination. And that evaluation would be done not in vRO by maintaining JS but as a simple CP assigned anywhere within vRA you like. There are a few ways to skin this cat, but if possible I like staying out of vRO as much as possible for things like this that should be configurable through vRA because it's quicker, easier, and you're not subject to the whimsy of vRO.