Is it OK to submit a poster version of an already accepted full paper to the same conference?

Some conferences explicitly encourage (or even require) poster presentation of papers. In this case, there is typically an option to simply request that your existing paper also be given a poster slot. Thus, it may in fact be quite reasonable to seek to present in both ways, if the conference supports this.

It would not, however, be appropriate to submit a separate poster paper that pretends to be different than the accepted paper. That would be self-plagiarism, and the conference organizers might look very badly upon you for doing that.

I would thus recommend getting in touch with the poster chair and asking if they allow accepted full papers to have an accompanying poster as well. If so, that's great, and you probably don't need to submit anything more than a formality at most. If not, then accept that there will be no poster and don't submit anything!