I don't think GLIMMIX generates a FitPlot directly, but you can use the OUTPUT stmt to create an output data set that has the predicted values, then use PROC SGPLOT to create the graph. The trick is that you need to specify what sort of predicted values you want from (BLUP or NOBLUP) and (ILINK or NOILINK). See the GLIMMIX doc for what these terms.
Here is code for an "S-like curve" that you can study and modify:
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