Re: Temporary class skills? Adding a class skill for one PC only
There are any number of traits that make individual skills into class skills. It might be more of a problem to find one that doesn't also give you a *bonus* to that skill, but there are several that do it specifically for Diplomacy, like Ease of Faith. If that works for you, just create a new trait and add it to the character.
Here's the text of Ease of Faith from an older version of PCGen (the one currently installed on my machine), by way of comparison:
Ease of Faith KEY:Ease of Faith_Trait CATEGORY:Special Ability TYPE:Trait.SpecialQuality DESC:Your mentor, the person who invested your faith in you from an early age, took steps to ensure that you understood that what powers your divine magic is no different than that which powers the magic of other religions. BONUS:SKILL|Diplomacy|1|TYPE=Trait CSKILL:Diplomacy SOURCEPAGE:p.4
I haven't done any PCGen coding in a long time, since I normally use Excel-based sheets these days, but I'm guessing it's that "CSKILL:Diplomacy" part that's important here.
If you DO get a bonus for Diplomacy at the same time, look through the trait listing at http://www.d20pfsrd.com/traits and see if you can just choose the one you need.