With the understanding that any board modifications of this type would instantly void the warranty... yes, this is possible, if you know what you are doing and are very careful. A good board shop could make the modifications for you.

The sampling on a DAP is dependent solely on the 20 MHz oscillator circuit on the DAP board. There is a separate CPU clock, but it affects the scheduling of system processing, not sampling activity. The 20 MHz oscillator could be replaced with a pin-compatible TCXO (Temperature Controlled Crystal Oscillator) with internal thermal regulator or temperature compensator. The other circuits will almost certainly will never know it, and will work fine. However, changing the frequency to anything other than 20 MHz is an almost sure formula for disaster – the propagation of timing signals through the board control logic is critically related to the 20 MHz time base.