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 toon the CMitems 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.