You refer to "stroke width" in double quotation marks, which makes it seem like your attribute table has a field called "stroke width." If that's the case, you can use this field in an expression, and scale the line width based on the value in the stroke width field.
If, as I suspect, you're actually talking about the stroke width box in the layer styling panel , then you can't do what you're asking for. QGIS just isn't designed that way. The value set in the stroke width box is not available in the expression builder. The Data-Defined Override setting will always override the value you enter into the stroke width box. See the second and third definitions of "override" from dictionary.com:
- to disregard, set aside, or nullify; countermand
- to take precedence over; preempt or supersede
What you can do instead is define a custom variable in the layer properties. Eg, you could call your variable
default_stroke_width. Once the variable is defined, you can use it in the expression builder with
Even though this variable has the words "stroke width" in its name, it's not connected to the stroke width box in the style panel. When you want to change the
default_stroke_width value, you'll have to go back into layer properties and edit the variable. This is probably less convenient than editing the expression itself.