Sorry Jeff I misread your original question. To define user libraries go to the following preference page: Window > Preferences > Java > Build Path > User Libraries. There you can add your own user libraries that will show up in the Spring Capabilities wizard.
As for your question about adding new Spring libraries to an existing project. This can be accomplished by the following:
– go to Project Properties > Java Build Path >
– select the Libraries tab
– Select the “Add Library…” button
– Select MyEclipse libraries in the wizard and hit next
– Select the libraries that you would like to add
Hit finish and your good to go! (Thanks to Support-Brian for this tip)