MS Project: How can I import tasks into a project that has already been started?

MS Project: How can I import tasks into a project that has already been started?

I am writing a feature to import tasks from an online bug tracking / project management tool into Microsoft Project, primarily for resource leveling. Currently I am importing all tasks in as Fixed work, and giving them an assignment to a single resource for 100% of the work. The duration is then dynamically calculated, which at import time is equal to the amount of work.