Sewer smell coming from Washer discharge pipe- Can i install a P trap?

My washer and dryer are in the basement. My sewer / waste line runs along the floor of the basement then goes through the wall and out to the main. The washer is at the end of the line. I have a 2" riser pipe coming out of the horizontal 3" waste line. Because there is no P trap on the riser or anywhere else on the waste line, I get sewer smells coming from the riser pipe. Can I add a P trap to the riser pipe to stop the smell or will the trap effect the drainage of the washing machine. Thanks in advance

ALL drains, including a washer, must be trapped and vented.

The harder part of doing it properly is to put a proper vent in it. If your location allows an AAV, you could put one of those in. In reality, where it is, it can probably get by without a vent IF EVERYTHING ELSE is plumbed properly, but it is still required by code.

IT is NOT a question of whether you "can or not", it is a health issue that you MUST do it. There are MILLIONS of washing machines with traps on their drains and they do not have any problems with it.