Let's start with the fact that you can't fuck the robot, because the nandroid series is catered more towards families with kids, unlike the maidroid series.
You can still have outercourse (the company is not liable for you getting pinched by any joint), or use the nandroid mouth, which doesn't come with any built in lubrication but it does have moderate suction capabilites. Please do not try to have your nandroid swallow any fluid. If it happens, please refer to the user manual for how to flush the vacuum line and pump.
Now, before we get to real intercourse, let it be clear that none of the following is endorsed by the manufacturer and it does void your warranty.
There are some ways to achieve penetrability exploiting the space inside the pelvic area, since the nandroid and maidroid pelvis is internally the same.
The easiest and cheapest one is to buy an aftermarket crotch-plate which can house a dumb onahole. This modification can be done even by the least tech savvy individual, as it only involves removing some screws to install the new part, but if you don't feel confident a certified technician can do this for you for a small fee. The onahole can be removed for cleaning, your nandroid will take care of that.
A step above this would be to install a maidroid sexual interface module, utilizing the existing connector shared by both models. Please note that this is a more invasive procedure since you will have to route the cable to the main control unit.
The module can be paired with a smart onahole to control temperature, contraction and lubrication (features will vary depending on the model you choose). You will need to install the control software from the maidroid series, which should not be a problem since your warranty is already void.
This is a vastly superior solution, but it will be much more costly since the interface is pretty expensive.