You can do this many ways.

When you say "parts" - I'm not sure if you mean operators or PARTs (as in 1-16). If the latter, you can use Part Volume as the destination and A.SW1 or 2 as the source. If the former (operators), you can use the carrier as the destination and target those operator levels. The carriers are the only operators that produce sound - so setting the level to 0 through an offset will mute that "branch" of the algorithm.

In FM-X there is not an XA-Control type parameter that would allow for you to turn off an operator for subsequent key presses while allowing the currently sounding (pressed) operators to continue sounding.

That said - you can still do something close. Use "Op AEG Offset" as the destination, A.SW 1 or 2 as the source, turn on all operators that are carriers, and set the ratio to -64 on a square curve that is aligned to the far right. This will make the levels "0" for the AEG which only affects the next notes and does not change the AEG of the notes you are currently holding. At least not the decay part. Maybe release will be affected since release hasn't happened yet for the note(s) you are holding.

For example:
1) Recall "FM Wr 1"
2) Touch PART 1 and navigate to "Mod/Control" -> "Control Assign"
3) Turn on "Auto Select", press A.SW1, then I delete the existing parameter I believe was Var Send
4) Add a destination "Op AEG Offset" (source should be A.SW1).
5) Curve type: Square, Polarity: Uni, Ratio: -64, Param 1: 9, Param 2: 2, Touch "Sw/Rate" and set Op 8 to "On" (rest of the operators can be "Off" ), Touch "Sw/Rate" again and set Op 8 to +7 (rest of the operators can be "+0" ).

This will apply the maximum subtraction to the AEG levels of Op 8 when A.SW1 is pressed ON and will leave the settings to default when not pressed. This will mimic an XA Control type AWM2 element switch. For "FM Wr 1" only Op 8 is the sounding carrier which is why I set to only offset Op 8. Other FM-X PARTs may have different operators (carriers) you want to target.

Hi Jason,

That's cool! Thank you!

I guess I also left off the option of turning "Kbd Control" off on all of the PARTs and manually selecting a single PART to play at one time. When you want to "turn off" one of the FM-X PARTs, switch selection to a different PART. This will also behave like XA Control of AWM2 under the condition that you have one active PART at a time.