It's possible they might - I remember last year that I bought a week's worth of fully comp cover under the terms of my Admiral policy for a courtesy car (was something like £17 for the week for a Ford KA).
So in theory I can't see why they couldn't come up with a price for someone to be added to your car for a week or so.
Probably best to give them a bell and see what they say though.
Yep, from memory its almost a fixed price. Went to put sister on Stilo for 3 weeks over christmas 2010, ended up being cheaper to add her as a named driver until policy renewal in May 2011.
No, they generally tell you if there's an admin fee, as its not really a change to my policy I dont think there is one. Also no charge for using credit card!
If you havn't taken this out already just see how much it is for the month, when my auntie had to share my car it was £65 for two weeks or for a month it was £70.