Follow the instructions on
https://prisonarchitect.paradoxwikis.com/Modding
you'll want to change the room settings in materials.txt, where you can find the following
BEGIN Room
Name Cell
Capturable true
Properties Housing
Properties QualityGraded
BEGIN Requirement Type MinimumSize x 2 y 3 END
BEGIN Requirement Type Enclosed END
BEGIN Requirement Type Indoor END
BEGIN Requirement Type Object Id Bed END
BEGIN Requirement Type Object Id Toilet END
BEGIN LikelyObject Id JailDoor END
BEGIN LikelyObject Id Bookshelf END
BEGIN LikelyObject Id Tv END
BEGIN LikelyObject Id Window END
BEGIN LikelyObject Id Radio END
BEGIN LikelyObject Id OfficeDesk END
BEGIN LikelyObject Id Chair END
BEGIN LikelyObject Id ShowerHead END
BEGIN LikelyObject Id Drain END
BEGIN Grading Type RoomSize Size 6 END
BEGIN Grading Type RoomSize Size 9 END
BEGIN Grading Type RoomSize Size 16 END
BEGIN Grading Type OutsideWindow Stackable true END
BEGIN Grading Type Item Id OfficeDesk END
BEGIN Grading Type Item Id ShowerHead END
BEGIN Grading Type Item Id Tv END
BEGIN Grading Type Item Id Radio END
BEGIN Grading Type Item Id Bookshelf END
BEGIN Grading Type Item Id Chair END
END