If you want to customize sisNET tasks - either adjust existing ones or replace all or some of them with new ones - make a copy of the SISNET_TOOLS.DGNLIB file and consider making modifications only to this file. To make the new DGN Lib available to all users, modify your SITE cfg file or your DATENBESTAND.SISCFG file by adding the following line:
SISNET_TOOLS_DGNLIB = path to you modified copy of SISNET_TOOLS.DGNLIB (e.g. SISNET_TOOLS_DGNLIB = $(_ustn_site)/sisNET/mysisnet_tools.dgnlib)
If you want to make it available only to one user modify its .UCF file in the same way.
Tips and hints:
Steps to customize your SISNET_TOOLS DGN library