Tune a single instrument

Hello,

For one of my current scores, I would like my string synthesizer to play an A4, but I want it to sound a bit sharper or flatter that a normal A, without actually being A# or Ab.

Is there currently any way to do this? By the way, I want this to pertain to one instrument in my score, and I already know about the "Master Tuning" feature but that's not what I want. Please let me know if you can tell me anything!