As long as you don't move any sliders or alike, LR is not applying any WB to the imported photos.
If it is RAW, then no WB can be applied. If it is DNG, then the settings done externally (in ACR for instance) will be imported and can be changed.
I do not know if there is any way, when you import a raw file Lightroom will apply its "Default Develop" settings and apply its WB as interpreted from the file.
Quote "my input RAW/DNG files are already applied white balance gain"
How has this been applied by the Camera's Firmware? Other RAW conversion software?
What do you see when you do import into LR, As shot?