Update your post to include full system specs and OS.
Include PSU: make, model, wattage, age, condition.
I do not know if you can connect a fan there or not. Likely dependent on the motherboard.
Configuration and physical connections matter.
Check the motherboard's User Guide/Manual.
However, do not apply a fix/solution without knowing the problem.
What is the desktop doing when you see temperatures rise and throttling?
Any other symptoms, occasional error codes, pop-up windows etc.
What do you see happening via Task Manager and Resource Monitor?