You can use Developer Studio to include an externally-managed taxonomy into your project, but you cannot alter the taxonomy within Developer Studio, even after importing it.
Ensure you are working with an externally-managed taxonomy, and not an externally-created dimension.
An overview of the process to include an externally-managed taxonomy in a Developer Studio project is as follows:
After you finish the tasks listed above, you can perform additional pipeline configuration that uses the externally-managed dimension, and then run a second baseline update to process and tag your Endeca records.