Are there any possibilities to counter the negative effects of low or micro gravity by increasing the atmospheric pressure?
Not all adverse effects of prolonged stay in microgravity environment, for example, it wouldn't make the slightest of difference regarding gravity gradient loss or decalcification, but it could increase atmospheric drag equally on all limbs and effectively help combat muscle atrophy. Problem is, it wouldn't be really practical, nor safe to additionally increase atmospheric pressure.
The mixture of breathable air would also have to use some other filler gas than nitrogen to prevent nitrogen narcosis, so perhaps argon that is equally inert. But regardless the filler gas used, increasing atmospheric pressure would additionally increase the load on the structure and further complicate extravehicular excursions (egress and ingress), and depending on the mixture used, might increase fire hazard.
Other, much simpler techniques with similar effects could be used instead, such as e.g. elastic compression garments, like ESA's Skinsuit, that offer resistance with the use of stretchable fabrics. They might be rather uncomfortable though, and still require strict exercise and dietary regime to help alleviate gravity gradient, body mass and blood volume loss.