You can add maybe 1-2 tablespoons to the soap nuts, depending on the size of the load... I wouldn't add both the soda and the soap nut liquid (or a liquid detergent) in the same dispenser at the same time because it could cake together too much and not wash out properly. You could try the fabric softener or bleach dispenser for the liquid. Or maybe just do one after the other in the detergent dispenser. I usually do that and often I'll just put in the washing soda, let it get washed into the washer. Then I'll put in the soap nut liquid, turn off the washer and start the cycle up again so that it starts another cycle of washing out the liquid in the dispenser. Be careful putting it directly into the machine, especially if using cold water, as it may not dissolve properly.
It'll definetly boost the cleaning power, help whiten a bit and leave a clean scent (nothing perfumy or chemicaly, just a natural freshness). You could also make a paste with the washing soda and use that to treat stains.