If your TPS supports composite plans (i.e. where all beams, and dose from all beams, are exported in a single RT Plan and RT Dose) then that's all you need to do.
However, there is at least one TPS (Eclipse) that cannot do this and must create a separate plan and dose for each prescribed region. In this case, you want to sum the dose files to create a single, composite dose file, and submit that. As for the plan, you will unfortunately just have to pick one of the plans (e.g. the breast beams) and submit that because a plan is required. We realize this will be an incomplete plan, but we do need something for your submission.
The other thing you should definitely do is contact the TPS vendor and lobby them to modernize their software to support this basic and essential feature, i.e. export a single, composite plan that has all beams, target prescriptions, etc.