A wall can be divided into two different functional categories at the same time.
If you have a wall sharing a portion as a partition with another wall and another portion exposed as an external wall, you can model it in OpenBuildings Energy Simulator.
In the image below, you could see a wall behaving differently in two different sections:
OpenBuildings Energy Simulator assesses the adjacency and exposure condition of a building element, especially walls, and assigns the functionality of the same. In the image above, you can see the wall having two different conditions; a portion is adjacent to the room beside it and the other is exposed.
To make the adjacent portion function as a partition wall, you would have to set the adjacency to a range so the software recognizes as a nearby element and sets the workflow accordingly. The rest of the wall, as not having any wall section nearby, will function as an external wall as usual.
To learn more about adjacency tolerance, refer to the article here.