I would say yes. bringing more people on to a project always creates more work, and I don't see any reason you shouldn't bill for that work.
EDIT: from Peter's comment below, and to expand on this thought a bit more.
I'm saying this assuming that since you are outsourcing, you are interfacing with the client directly. And by that mean handling all invoicing and financial matters along with most if not all client communication. That would also mean that you are paying the outsourced person directly.
If this is the case, then the subcontractor's client is you , not your client. That means, among other things, that you are responsible for his pay, not your client (even though your client is indirectly helping you cover that). In that case, the percentage you take off the top is your fee for handling the client relationship and assuming all the financial risk and responsibility that comes along with that.
Having said that, your management tasks for the project, (which I'm assuming is mostly communication between you and the client and you and the subcontractor) is a service that you are providing your client. And as such, I will bill it in the same manner as you bill any other services to said client.