In order to see a picklist in more than one roadmap item, create a new Lookup in Classic UI, then create an attribute in the Roadmap Item object associated to that lookup. Ensure that an API Attribute ID is entered for the attribute, or the attribute will not show up as a Picklist.
See the Create a Global Custom Picklist
section of the New User Experience: Top-Down Planning with Roadmaps
documentation for more details on how to do this.