You need to have your assembly transition in width by creating and targeting an entity like a polyline to define the extent of the widening. Once you create the polyline then you can select it as a width or offset target through the Corridor Properties > Parameters tab. The subassembly that your using needs to support transitions.
I suggest that you look at the following tutorial to get some background on creating a transition:
Tutorial: Creating a Corridor with a Transition Lane
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