It is not quite clear what exactly you are trying to do. If you have an external Matlab m-script or function, you can use it as boundary coefficient like it is. For example, if you have a function saved as named Temp_roadway.m which takes x and y coordinates as arguments you can just use it as "Temp_roadway(x,y)" in equation and boundary coefficient expressions. This example illustrates how to implement a mixed Robin FEM boundary condition with a coefficient expression.
If it is just a constant or variable you and want to import it into the Gui, then you can use the "Import" > "Variables From Main Workspace..." option from the "File" menu (FEATool has its own internal memory workspace so you need to import variables from the main Matlab workspace if you want to use them there).
