Perhaps the most effective way of building soft skills in new generation project managers is to enable them to learn on the job at the point of need. It is often not possible to release project managers into two-day training courses.
Instead, give them access to a learning module on conflict management or negotiation and the ability to access that when they need it, even from mobile devices while on the move or at client sites. Backstop this with coaching and mentoring and the learning that results is likely to be even more successful.
Organisations who display willingness to invest in their own people to develop them into effective project managers, rather than attempting to buy in project management expertise, will see the added benefit of higher levels of engagement in their own staff.
If employees feel their skills base is valued and that they are likely to be able to develop these skills in order to be promoted to more senior levels, they are surely more likely to want to stay at the organisation. The organisation will then reap the benefits of a well-qualified and stable workforce delivering successful projects.