In LPX it is almost that easy.....
Note: Make sure you have turned on ALL the advanced features in LPX's Prefs/Advanced before proceeding
1) Select the track you wish to automate and make sure the plugin (or in your case, I assume the Pedalboard interface) is visible on the screen
2) Press Command + L to open the Control Assignment window.
3) If the Learn button in the bottom righthand corner of the Control Assignment Window is not Blue.. click on it once to turn it on (Blue in color)
4) Click on the wah pedal graphic in the pedalboard window
5) Move your Expression Pedal all the way up and all the way down. You should see the Wah pedal graphic move along with your movements of the expression pedal.
6) Click the Blue Learn button to complete the process.
7) Done.
Additional info: Instead of... or in addition to...your expression pedal you could also assign any other slider or knob on your midi controller for example to any other knob/slider or parameter in the pedal board or any other plugin or the LPX mixer functions such as pan or effect send level... for that matter... you currently have active too... by repeating the above process from Step #2
Now read up on automation in the manual as to how to best record such movements and be able to easily edit them if needed.
http://help.apple.com/logicpro/mac/10/#lgcpb1a6ab26
...and also type Automation in the search box in the online guide for more info...