You can't AFAIK. You would have to bounce/render the effects right onto the clips (before making your x-fades) then create your fades on the new clips.
But... I have never ever had to do this. I know what you are thinking but an X-Fade should not be causing any decrease in signal feeding the sim. X-fades, in theory, result in a smooth/even transition between clips without any volume reduction.
Now Fade Ins and Fade Outs are another matter. That WILL reduce the input level going into your sim which can be undesirable (less input signal means the sim/amp isn't being driven in the same way so the distortion might get stoopid or tone gets lost or, or, or...).
In THOSE cases you can solve this by
a) bouncing the effects directly to the clips as previously suggested or
b) using post effect level automation (like the track fader*) to create the fades instead of the fade handles on the clips. This is likely the better option and keeps full input signal going to the amp sim (so there is no change in input level going to the amp sim therefore there is no change in the ouput of the sim).
*I prefer to put a Prochannel module at the very end of the signal chain right before the track fader and use that for the Volume automation. A blank/empty FX Chain Module** works great for this purpose. You create and insert the blank FX Chain Module (which will not have any effect on the sound of the track). Then you automate the Output level fader of the module thus keeping your Track Fader free from being overtaken by automation (so you can still use it as an overall track volume control).
Cheers.
**Credit for the PC module automation trick goes to Craig Anderton (from his "Advanced" video series) and credit for the FX Chain module variation of the trick goes to scook. Craig showed it using the Tube PC Module but I think the vid predated the introduction of PC FX Chain modules. scook suggested an empty PC FX Chain here on the forum. Just had to give props where props were due. Extremely useful technique.