Social Security income is not compensation needed to support making a retirement contribution. You can only make a retirement contribution if you have compensation from employment, generally either some amount in box 1 of a W-2 minus any amount in box 11 or net profit from self employment.
No, you can't have 100% of your Social Security benefits withheld for taxes. Tax withholding is calculated on the amount of the benefit payment at a rate of 0%, 7%, 10%, 12% or 22% only. I would expect this to be on the gross amount before subtracting Medicare payments, but I don't have specific knowledge of that.
