16 U.S.C. § 441k — Acquisition of property for park

(a) Consent of State or Oglala Sioux Tribe of South Dakota; transfer from Federal agency. Subject to subsection (b), the Secretary of the Interior may acquire lands and interests in lands within the park's boundaries — by donation, by purchase using donated or appropriated money, or by exchange. There's an exception: any land or interest in land owned by the State of South Dakota, one of its political subdivisions, or the Oglala Sioux Tribe of South Dakota, can only be acquired with that owner's consent.

Separately, no matter what any other law says, land within the park that is under the administrative control of some other federal agency can be transferred to the Secretary of the Interior's control without any transfer of funds needing to happen.

(b) Easements. For land that's inside the park's boundaries but outside the boundaries of the gunnery range described in section 441l, the Secretary of the Interior is limited in what can be acquired: only rights-of-way and scenic easements — not full ownership of the land itself.