I am not sure how you intend to add the new variable to wrfinput, e.g., modify wrfinput, or make it available in met_em and then run real.exe to include it in wrfinput?
If you simply modify wrfinput, then you don't need to worry for REAL program. If you include this variable in met-em, then you will need to modify
dyn_em/module_initialize_real.F to process this new variable
In either way you needs to modify Registry so that the model will know that this is a variable included in wrfinput.
Another important code is phys/module_physics_init.F, which should be modified to obtain the new variable. Then you will also need to modify other related codes such as module_sf_sfclayrev.F to pass this variable to the specific schemes you use.
Please look how U10 is processed , which serves as a good example for your case. This is not a trivial task and it involves lots of code changes.