Jimmy
First, a little history in case you or other interested folks are not that familiar with the Motif lineage. The original Motif-6/7/8/Rack "Classic" flagships were released in 2001 to replace the earlier "EX" units. They had provision for mounting 2 aftermarket WAVE-ROM expansion boards, but had no provision for downloading wave samples. The Motif ES-6/7/8/Rack models replaced the "Classic" units in 2003 and were the first models to offer provision for optional aftermarket wave sample memory. Shortly after that, the MO-6/8 models were released as "lite" or economy versions of the ES units. These were the first of the Motif "lite" models. This would be the MO-6 that you are asking about. The ES units were replaced by the Motif XS models in 2007, with the MOX-6/8 being the "lite" versions. The latest Motif XF flagship models were released in 2010, with the MOXF-6/8 being the "lite" models. All of the full-blown flagship models, with the exception of the original "Classic" units could be equipped with optional (aftermarket) memory for the download of expansion wave forms (samples). Typically, the "lite" units shared identical sound engines and wave-rom and most of the voices, and effects with their flagship counterparts. Cost savings came in the form of lower cost keybeds, displays, body shells, a few fewer voices, less polyphony, lack of LAN connectivity, and no provision for optional wave form memory, excluding the latest MOXF-6/8 models. They are the first of the "lite" model lines to offer optional flash memory to hold 1 Gb of downloadable wave samples. Use of lower cost A/D converters and audio circuits (pre-amps and amps), in the "lite" units was always suspected and much discussed, but to my knowledge, was never confirmed.
Now, back to your question. NO ! The MO-6 has no provision for optional memory to hold downloadable wave samples. Its only wave source is the permanently burned factory WAVE-ROM. To get into downloadable wave samples in the Motif line, you would need one of the full-blown flagship ES, XS, or XF models or one of the new MOXF units. BUT . . . as with most ROMplers, while the wave samples are fixed in ROM, the actual voices that use those wave samples can be tweaked and edited, and those tweaks and edits off-loaded and ported to other like units, so you are not completely paralyzed when it comes to new or different "sounds". While the "motifator.com" forum is the home of the Motif guru's, I really didn't see much there in the way of downloads except for the utility type stuff - editors, firmware upgrades, etc. Either the Motif folks don't do much tweaking/editing, or they don't upload their work to share with their colleagues, but they do have a "pay" sound library that has a fair selection of new sounds (patches) for the MO-6/8 that you might want to take a look at:
http://shop.motifator.com/index.php/voice-libraries/mo-voice-libraries.html
Best of luck !