Is it possible to save custom variables so they are loaded into any CFD Post session?
Yes it is possible.
In your current CFD-Post session, define expressions and make sure they are working.
You can save this State File for further use.
Then delete all the contours, isosurfaces, etc. and leave only expressions that you want to save for a different session.
Save new State file.
In your new simulation, you should be able to load this State file which will bring all these expressions.
Though not recommended, you can even edit this State file and only keep expressions section and save it under different name:
# State file created: 2011/04/25 14:25:11
# CFX-13.0 build 2010.10.03-23.02
# everything deleted prior to this
Accumulated Time Step = 120
Current Time Step = 120
Reference Pressure = 1 [atm]
Sequence Step = 120
Time = 0 [s]
atstep = Accumulated Time Step
ctstep = Current Time Step
ptotabs = Total Pressure + Reference Pressure
sstep = Sequence Step
t = Time
# everything deleted after this
You should be able to load this State file in a new session.
Please note if you are using any variables that include named selections, it won't work unless those zones are available.