First, make a DDR double chart in DDReam Studio and save it as a .sm file.
At the top of each double chart, there should be a couple lines like this:
#NOTES:
dance-double:
Replace "dance-double" with "techno-single8" and StepMania will proceed to load the chart as an 8-panel techno chart.
You can also rename "dance-single" to "techno-single4" if you want to play 4-panel charts without switching gametypes.
The arrows in techno-single8 mode are laid out as in this picture:
and StepMania will map each column from the double chart to the chart with the same position in the single8 chart (so P1 Left = DownLeft, P1 Down= Left, etc.).
StepMania 5.0.9 includes a techno mode noteskin so you should be able to just select "techno" from the Select Game menu and go. Older versions require you to get a techno noteskin first.