How to setup iTunes account in a business environment and can you share app purchases?
Below is the Apple license agreement for the Mac App store, accessed on 24 January 2012.

According to a layman's reading of the text (I am not and do not pretend to be a lawyer), what you describe should be both possible and allowed under the terms of the license.

You or your IT staff would log in to the Macs, authenticate to the Mac App Store with your  company Apple ID, install the App(s), then sign out of the Mac App Store.  Your employees could then log in with their personal accounts and install any Apps they may have purchased, although according to the text in section i, they shouldn't do company work with their personal apps.

I hope this is helpful.

From http://www.apple.com/legal/itunes/us/terms.html


  MAC APP STORE PRODUCT USAGE RULES
  
  Except as otherwise set forth herein,
  
  (i) If you are an individual acting in your personal capacity, you may
  download and use an application from the Mac App Store (“Mac App Store
  Product”) for personal, non-commercial use on any Apple-branded
  products running Mac OS X (“Mac Computer”) that you own or control.
  
  (ii) If you are a commercial enterprise or educational institution,
  you may download a Mac App Store Product for use by either (a) a
  single individual on each of the Mac Computer(s) used by that
  individual that you own or control or (b) multiple individuals on a
  single shared Mac Computer that you own or control. For example, a
  single employee may use a Mac App Store Product on both the employee’s
  desktop Mac Computer and laptop Mac Computer, or multiple students may
  serially use a Mac App Store Product on a single Mac Computer located
  at a resource center or library. For the sake of clarity, each Mac
  Computer used serially by multiple users requires a separate license.
  
  (iii) Use may require sign-in with the Apple ID used to download the
  Mac App Store Product from the Mac App Store. Mac App Store Products
  can be updated through the Mac App Store only.