The Smart Crystal Pro polarization modulator from a new (to us) company called Volfoni (which also makes both active and passive 3D glasses) can take your single chip active glasses DLP 3D projector and turn it into a passive glasses 3D projector. The brochure says it can do this for any 3D single lens projector, but the device has only been verified effective on DLPs.
What it does, in effect, is to substitute rapidly alternating polarization of the filter, which is positioned in front of the lens, for the switching of active glasses. You'll need a connection from the sync transmitter output in the projector to the Crystal Pro to make this work, or alternately DLP Link in the projector (DLP Link is a process that uses rapid interframe bursts of light from the DLP projector that normally triggers active glasses). (If your projector has neither, that is, no DLP Link and an on-board sync transmitter, it will not work, as I understand it. You'll also need a screen that preserves polarization. Estimated cost will be $1500 for the Crystal Pro itself, or $2000 for a package including both the Crystal Pro and a slide that can automatically move the polarizer out of the way for 2D material.