Why HMS Unicorn could not simply gain more machinery

Could HMS Unicorn have been designed for 30 knots by adding boilers and turbines?

Not simply; the change would require a third shaft and conflict with the treaty-driven design choices behind the ship.

Reaching 30 knots would require more than boilers and turbines; the hull would need a third shaft and associated redesign. Unicorn had deliberately been shaped to fit an auxiliary treaty category rather than consume carrier tonnage, illustrating how legal definitions could constrain later operational ambitions. The ship nevertheless proved faster than her nominal design expectation, but that did not turn a two-shaft aviation-support vessel into a fleet carrier. Plans for additional ships of the type existed, yet economic restraint and efforts to avoid advertising a wartime timetable prevented the Royal Navy from realizing them before the conflict.