This is a partial answer, and I think it would be good to have confirmation from someone who knows more about this than I do.
After my previous answer I did some more searching, and found History of the SGP4 Propagator on the STK website.
It seems that this happened a lot earlier than I'd originally thought!
In the mid- to late 1970s, the SGP4 was modified to address deep space requirements. The incorporation of deep space algorithms into the SGP4 was developed primarily by Air Force Captain Bruce Bowman and Richard Hujsak of the 14th Aerospace Force/Air Defense Command/NORAD. The current SGP4 propagator, and then, is really an SGP4/DP4 propagator."
I assume that the SDP4 deep space correction is automatically implemented for TLEs with mean motions of 6.4 per day (225 minute period), and that there are no settings or controls to turn this on or off. TLEs are generated expecting standard SGP4 behavior, so my guess is that this all happens hidden from view and you don't have to turn anything on or off.
But I'd feel better if someone were to add to this answer or write a new one with confirmation.
STK's page SGP4 Propagator says:
By default, STK utilizes the CSSI SGP4 routine, Version 2008-11-03
I don't know what version that is exactly, but for
CSSI see http://www.centerforspace.com/downloads/