Medicare allows you a special election period to make a change while in a skilled nursing facility; it begins the first day your wife is in the Skilled Nursing Facility and ends 2 months after her discharge from the skilled nursing facility. However, that special election period only allows enrollment onto a Medicare Advantage Plan or onto a stand-alone drug plan.
If she elects to go onto a stand-alone drug plan, it will terminate her current Medicare Advantage plan and place her onto Medicare Part A and B. To go onto a Medicare supplement, she would need to pass the underwriting for a supplement carrier and could be declined. Please be aware that if she is unable to pass underwriting and has only Medicare Part A and B for her medical coverage, it leaves you with financial responsibility for all the copays/coinsurances that Part A and Part B do not cover.