A judgement will need to be made, depending on the severity of hyperkyphosis, of whether the supine exercises are appropriate to start with.
Propping with small pillows, or towels under the head and shoulders so that the starting position is a close to aligned as possible AND that there is enough supporting surface to push against.
Should the hyperkyphosis be severe, it may not be possible (or preferable) to choose that particular exercise even with propping.
For example, if the head cannot be aligned with upper spine, the head press would turn into a head extension exercise which doesn’t focus on only the erector spinae (which is the point).