Could you clarify your post? If you paid all cash, what mortgage interest are your referring to?
the coop features I am awaare of are -- you pay cash for buying it and then through the year you pay a monthly fee to the cooperative towards the wrap-around mortgage ( that the building has ) and the property taxes that are paid on the whole property. Then at the end of the year you get a statement that tells you the percentage of you monthly amounts allocated to the mortgage interest and to the prop. tax. You sue these percentages to come up with your contribution to the deductible mortgage interest and to the property tax. That is how it works in MI
Payments on the co-op mortgage, to the extent attributable to your ownership, are deductible mortgage expenses (an itemized deduction) the same as if they were on a mortgage you used to purchase your interest.
Mortgage interest, refinancing, property taxes and mortgage insurance are all under the Deductions and Credits tab for "Your Home."