There is a new version of DTSA-II at
https://www.cstl.nist.gov/div837/837.02/epq/dtsa2/index.html. It supports custom windows. It is a little bit clumsy but it can be done.
The trick is to find the file epq.jar in the install directory (usually something like "C:\Users\username\AppData\Local\NIST\NIST DTSA-II Microscopium 2021-01-06"). Open this file up with a ZIP file reader like 7Zip and navigate inside the JAR file to "epq.jar\gov\nist\microanalysis\EPQLibrary\Detector\". There is a CSV file here called "custom.csv". Extract it and edit it to represent the efficiency you desire. The first column is the energy of the low side of the bin in eV. The second column is the efficiency (1.0 => 100% efficient, 0.5 => 50% efficient, etc). Finally replace "custom.csv" in epq.jar with the edited version. DTSA-II will now use this data for the "Custom Table" window type.
You can verify this from the command line by using "listDetectors()" to find the "d??" variable associated with the detector (say "d53"), then using "display(windowTransmission(d53))" to show the window transmission in the spectrum display (100 => 100%).
If you update DTSA-II, you'll need to update "custom.csv" in the new "epq.jar".
Does this help?
Nicholas