Thevenin equivalent in a nonlinear circuit
Yes, it is possible to use the Thevenin equivalent of a voltage divider driving an emitter follower.

The subcircuit you are replacing by a Thevenin Equivalent is a linear circuit (just the part consisting of 15V constant voltage source and the voltage divider; you are not replacing any of the non-linear part). Therfore using the Thevenin Equivalent for the subcircuit is not only an approximation, it is completely equivalent and perfectly ok. It doesn't matter that that subcircuit is connected to a non-linear subcircuit. 



If you would put both implentations of the linear subcircuit each in a black box, there'd be no way to distingiush them from outside; not even by connecting an external non-linear circuit like your emitter follower. That's why it is called Thevenin Equivalent and not Thevenin Approximation. 

What you can not do (in general) is to replace the non-linear subcircuit (right part with transistor) by a Thevenin Equivalent. There isn't a definition of a Thevenin Equivalent for a non-linear circuit.