In that situation, I'd suggest:
Acquire the skills through a contract based relationship. Pay someone (or company) a very high $/hr rate so you can immediately get work done in the subject area needed. At the same time, make arrangements for your senior in-house talent to begin acquiring the skills (for example - involve them in the project so they are sitting w/ the contract resources).
Hiring someone fresh out of school and paying them $$$ more than your senior developer isn't going to work. The HR issues described in this article are significant, deep/impactful, and will easily have consequences beyond this single project.
Your staff understands that IT management will regularly look for people outside the organization to fill niche roles (i.e. you may need to hire an Exchange 2010 consultant to help you with the ugprade even if you have a decent "jack of all trades" admin in place). They know that consultants typically are well paid but that they won't be around long.
If the team members can't pick up the skills (i.e. the candidate is truly extraordinary or the skills are that niche), then things may be different ...
BTW - reading between the lines of this story gives me the feeling that the developers in question were not keeping their skill sets polished and that this may be a bigger root problem that needs to be addressed. If the organization's developers are all client-server and the organization thinks its going to need web/mobile/etc developers then it needs to send the message to its developers that they need to acquire the appropriate skills, give them the opportunity to do so (i.e. training, work projects, etc), and get rid of them and replace them with people who are committed to evolving with the technology.