Create a custom property from 2 properties definitions ?

Hi,

I'd like to create a dynamic custom property from 2 properties definitions manage by the property dictionary but  I don't find howto aggregate each value of properties definitions ?

For example, in my blueprint :

DT : CPT or DA (dropDownList, prompt user yes)

OS: Lin6 or Lin5 (dropDownList, prompt user yes)

I want to update custom property (Linux.externalScript.Path) with values selected from the user.

So that's mean, I want a custom property like that : Linux.externalScript.Path = CPT-Lin6 if user select CPT and Lin6

Is it  possible ?