My question is how to include issues into roadmap planing that take 10 days, but only require 5 days of real work because there is some external dependency. As a consequence the engineer only needs to be blocked by 5 days, but the whole issue needs to be planned with 10 days. Any suggestions?
Yep, the sub-task doesn't need to be assigned, but if you are likely to use it for reporting then it would make sense to have an 'external' user/team rather than just being unassigned.