If the lenses are not currently made to mount on Nikon DSLR bodies (i.e., Nikon F-mount), then no, you probably cannot buy an adapter to use them on your Nikon body.
The reason for this is that Nikon's F-mount has one of the largest flange focal distances (also known as registration distance) for currently-available DSLR bodies on the market. I believe your Samyang lenses are available for the following mounts: Nikon F; Canon EF; Canon EF-S; Minolta/Sony A; Pentax K.
The Nikon F mount has a 46.5mm flange focal distance, larger than all of the other mounts listed above. So if your lenses are not F-mount ready, then they are physically too long to be used on Nikon bodies, and still be able to focus at infinity.
However, if the lenses were made for Nikon F-mount, then in terms of flange-focal distance, they could in theory be mounted on any of the mounts I listed above, or any other mount that has a flange focal distance of less than 46.5mm. In other words, Nikon F-mount system is the "universal lens donor", whereas something with a relatively tiny flange focal distance, like the Sony E-mount (18mm) or the Nikon Z-mount (16mm) would be almost a "universal lens receiver", in that most DSLR lenses can be adapted to mount on it.