is it possible to runAs(..) with a temporary set of roles/permissions?

[Jason]

hi all,
  related to my previous post, I am trying to use Shiro's runAs functionality 
to 
assume an identity, but I'd like to specify a new temporary set of permissions 
for 
the assumed identity. 

I want something like: 
userA canRunAs:userC:canDoX
userB canRunAs:userC:canDoX,canDoY
UserC permissions depend on who is assuming the role

userA.runAs(UserC)
permissions=canDoX

userB.runAs(UserC)
permissions=canDoX,canDoY

I cant quite see how to achieve this, any help greatly appreciated.
Thank you.