Planning for unforeseen risks mainly means adding contingency (padding the project schedule and increasing the project budget) to deal with these risks.
Naturally, unforeseen risks catch the project manager by surprise, so what should he do (other than adding contingency) to deal with them? Well, one thing that he can do is to add, in the risk management plan, a clause that addresses the unforeseen risks and have the client sign on it. The clause should clearly state that if the project faces a risk that is not listed in the RMP, then here's what should happen... (the three dots should be replaced by something like the "client should accept the extra delays and costs incurred by these risks" or something similar).
Note that another good way to plan for unforeseen risks is to compare your project to similar projects in the past and to see what unforeseen risks these projects faced, and what was done (or should have been done) to handle these risks.