A workaround might be to use Tools > Sketch Tools > Face Curves and then select your surface, select a vertex of the edge you wish to negatively extend. Uncheck either the blue or pink field depending on which runs in the direction of your selected edge.
Then you need to enter a percentage in the still checked field (so no "mm" distance terminology unfortunately) and it will create a 3DSketch. You can now use Insert > Surface > Trim to use the sketch as a trimming tool.
The neat thing is the 3DSketch will get consumed underneath the Trim so you're left with just one item in the tree just like an Extend would.
If you go inside the 3DSketch and select the curve, at the bottom of the PropertyManager you can see the percentage value and edit it at any time you wish. Also if you change your surface around, the 3DSketch will update and so will the Trim that is using it as trimming tool.
I like the suggestion!
Vote on this ER:
SPR 542636: allow selection of points/surface or the input of negative values for the distance end condition of the surface extend feature to trim or shrink a surface
I tried, I couldn't find it in either the SPR list nor the ER list?