You can set the initial condition for a node in the circuit. Search for initial conditions in the spectre manual or whatever simulator you are using.
However, it doesn't make sense to do it for a DC sweep since DC doesn't simulate the dynamic characteristics of a circuit. I am guessing you are looking for which clock voltage will cause the output to change. You could set the output initial voltage and run a transient simulation with the clock input being supplied with a slow ("nearly DC") voltage ramp. It just needs to be much slower than the clk-to-q time.