You can do this either by using a Thru machine on an OT track or, if you dedicate one OT track as Master Track, the Master Track’s effects will affect the other seven tracks and the external input signal. If you just use the OT’s mixer you will not be able to use the OT’s effects.
If you had an external MIDI keyboard, you should connect the keyboard’s MIDI Out to the OT’s MIDI In, and then the OT’s MIDI Out to the external synthesizer’s (NR2X’s) MIDI In. After careful setting of the MIDI channels and settings on all the units, this will enable you to play the OT’s internal audio tracks and the NR2X sounds.