Yes, it is possible modifying fields via the TOM.NET API or Core Service inside a C# TBB. 

If you use TOM.NET for this instead of Core Service, you can not do any write operations on the items being rendered/published within the same publish session.  There is also a setting in Tridion config that you have to enable to allow writing from templates (I can't remember which one ATM).  Best is to use Core Service for this.