Very useful content here as it addresses a very common question when setting up such simulations!
Indeed for an accurate representation of the total wall resistance, one might need to incorporate all of the layers that constitute the wall including the internal and external air films.
In case those additional resistances (such as of air or other wall layers) were not accounted for in the U-value, then one can incorporate these additional resistances as suggested by the forum post
here.
The information there is complementary to the one in this post. It simply touches on how to take into account all of the resistances of the wall layers (R-value which is the inverse of the U-value) and then converting it to a thermal conductivity value based on the thickness as demonstrated here.