Making a substitution for the server

Question
Tom,
Can you substitute for a player who is currently serving?

Thanks!

Answer
Good morning and welcome to www.allexperts.com.
I've never been to an event (h.s., club, NCAA) where putting in a sub for a girl during her term of serve was illegal.  If someone told you it wasn't legal, I'd like to see that in black-and-white.  
I say this so emphatically b/c I've been coaching & reffing using the National Federal rule book since 1985.  I've been coaching club vball since 1991 using the USAV rule book.  In every year, in every season, etc. what you described has been legal.  
What else can we do for you?  If you can think of anything, please contact me anytime.
And I hope you have a moment to visit us at www.coachhouser.com.  Last summer we directed 11 site camps.  In the past 4 summers, we've directed 38 in South Carolina, Alaska, Penn, Delaware, etc.  We can't wait for summer 2013.  
Hope you're having a super weekend!
Coach Houser