Is associative ligand exchange possible with octahedral complexes?
It could — and there isn’t even a general restriction on the electron count.

Any pentacoordinated species can easily be expanded to a hexacoordinated one. It is just a question of the ligand approaching from the correct angle. In fact, I would even want to prefer associative ligand exchange for pentacoordinated species over dissociative since there is less reorganisation involved (only two ligands need to shift positions compared to at least three in dissociative).

Also, it should be mentioned that the number of electrons in each structure does not have to be 18 or less. Many (pseudo)octahedral complexes such as $\ce{[Co(H2O)6]^2+}$, $\ce{[Cu(NH3)4(H2O)2]^2+}$ or $\ce{[Ni(H2O)6]^2+}$ have more than 18 electrons and are fully comfortable with it.