Put dsm.opt and the msi in a folder together. Create a new msi package in BBSA and point to that folder as the source.
To add the value to the file do something like:
echo "name=value" >> ??SOURCE??\dsm.opt
You will also need to modify the msiexec line to be something like:
msiexec <options> ??SOURCE??\<nameofmsi>.msi /RUNONCE /etc /etc
Throw another line in the Install command box to cp the file to the intended destination that says something like:
cp ??SOURCE??\dsm.op "C:\Program Files\SoftwareInstallFolder\config directory"