Yes, it can be, though a true nation-state would have much less of a need to be federal than other states would.
A federal state is generally needed when the country is either A) very big or B) diverse in ways that coincide with geography. A nation-state should not be as likely to be diverse because it would not be like (for example) Nigeria, with one religious group in the North and another in the South. However, a nation-state could well be large enough to need federalism and could be diverse in non-ethnic ways. It could, for example, have farming regions and industrial regions that see themselves as distinct from one another economically and which have different political leanings but which still see themselves as one nation.
So, the fact that a state is a nation-state does not mean that it cannot be federal. However, it would be less likely to need federalism than a state that is not also a nation-state.
Wouldnt the definitions be conflicting?
But if you look at Ronald Watts' definition of federalism: "a combination of regional self-rule and shared rule" with the classical idea of a nation-state, that is sovereign political entity that covers the same area as a cultural (ie a people) unit of the same "size".