You can by not choosing the option from the first screen (it worked when I tested it on the bucks game just fine earlier in the day) but by doing it from the "Teams" tab within the betbuilder when you are selecting your picks.
It allows you to toogle between which point spread you wish to put together with your first selection of the ML winner of the match.
Which is including OT.
Select your total points over or under and your done with building your bet.

note: Tested some variables for all matches in the nba tonight except for one which does not have the betbuilder option available, and you are able to select a winner, O/U & spread (handicap of points) on a team under the teams tab.

Harden going to the Nets is almost a done deal after the embarrassment against the lakers last night.