While we do not currently support backfilling the Base, Build, and Specialty phases in one single step, we do support the ability to schedule each individual phase according to its end date.
When adding your Specialty Plan to the Calendar, you can schedule it to end on the date of your A race by entering your race date into the “End Date” field. It will then automatically backfill the Specialty Phase according to that end date.
You can then repeat this process for the Build and Specialty Phases 
I hope this helps!