Hi @aatw,
If you need to enforce more or less strongly the presence of middle values for factors, simply use the Discrete Numeric (3 levels) or Categorical (3 levels) factor type. If you use categorical factor type, you can switch the factor type back to numeric continuous after design generation (or use the Convert Labels to Codes utility to switch the nominal values quickly to continuous).
Here are the comparative correlation maps results with the original design, a D-Optimal supersaturated design (with 3-levels discrete numeric factors) and a D-Optimal supersaturated design (with 3-levels categorical factors) :

Attached you'll find the designs compared with the scripts for the correlation maps.
See similar discussions about enforcing a specific number of levels for DoE factors :
DOE with 3 levels for continuous factors
Inquiry about Experimental Design with JMP
Number of factor levels in I-Optimal design
You can also look at Group Orthogonal Supersaturated Designs if you are in an early screening stage.
Hope this complementary answer will help you,
Victor GUILLER
"It is not unusual for a well-designed experiment to analyze itself" (Box, Hunter and Hunter)