If you are editing symbols within xDX Designer you should not need an additional license. That would only be required if you wanted to use the xDM Library Tool stand-alone.
I'm not an expert in the use of the tools, so I can't speak to the various ways you might be able to create symbols, but anytime I've done it for testing purposes I've use the xDM Library Tool (Tools > xDM Library Tools). When invoked from within xDX Designer, it doesn't need an additional license.
If that isn't what you are looking for, then you may want to open a service request or post on the xDX Designer community.
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