You are creating an independent library which does not collide with the existing techLib definitions. You may have to map the additional layers in the ITDB library to the layers in your process techLib using a layerMap file for GDS and equivalent layers for the tools. This leaves open many potential areas of error.
If the devices are not complex, you could represent them as pcells with the layers and possibly the rules defined from the existing techLib. Each placement would be custom for the design. Use dbWriteSkill to get the initial SKILL which you would have ot parameterize with layer names, etc.
You can also use the mapper capability to take an original library and map it to a specific technology. Copy the original library, set up the mapping table (once per technology) and it will change the layers to the ones in the techLib. You will now have a copy of the original library in the new technology. This is probably the safest methodology.