I now have a Nikon D3200 and the lens is not great. Can I adapt my Sigma Aspherical IF 28-300mm zoom lens, which I'm currently using on a Pentax MZ-30?
The answer is yes, but you might not be happy with the results. You can use an adapter like this Fotodiox K-mount to Nikon adapter, but there are several drawbacks.
Overall, you are probably better off buying a new lens.
There are two important restrictions for adapters from one system to another. First the lens must be made for a sensor or film format that is bigger or the same than the target format. If I am correct this is the case here. The second is the flange focal distance, the distance between the bayonet and the sensor. If this distance is higher for the camera system than for the lens system you want to adapt then the back end of the lens can't get close enough to the sensor. This means that the optics don't work as calculated; in practise this means you get a macro effect and you can't focus on infinity anymore.
As you can see the flange distance of Nikon F (46.50mm) is higher than Pentax K (45.46mm). This means that an adapter needs an additional lens to correct this. The additional lens has two effects: Higher cost and lower quality.
Plus depending on your camera system and the specific lens you may lose comfort, e.g. for old mechanical lenses you often can't use the automatic aperture and thus can't use automatic and some semi-automatic modes. I am sorry, I can't make a statement about this specific combination you want to use.
But yes, you can adapt this lens, such adapters exist. Now you know the search terms: "nikon f pentax k adapter".
But: I don't know what lens "is not great" because you don't name it. A Sigma 28-300 with adapter probably will deliver less than stellar quality. And it will be too long at the short end to be a useful lens for general purpose, just useful as telelens. You have to check the prices to see if you want to spend the money for an experiment, but I wouldn't expect much gain in terms of quality. IMHO adaptor only pay for very good lenses, for technical experiments or when lenses are not available or very expensive for a system. If the price for used lenses is nearly the same I would just swap via selling and buying.