You can do MSK with rectangular pulses to I and Q.
For two state FSK you put the proper triangular waves into I and Q to make the vector rotate clockwise or counterclockwise. (advancing phase or retarding phase)
I deduced the triangle method from a thought experiment. I have no practical experience doing it in hardware.
[added next day] After more thought, you use sine and cos instead of the triangles. I is cos wt and Q is sin wt. Where w is the frequency shift. For shifting the other direction use -wt which changes the sign of sin but not cos.
You can generate these with two integrators in a loop with some sort of amplitude limiting control.