" Is it correct that before files can be read by Matlab, the folder where files are located have to be added to Matlab's search folder using: "
No, that is incorrect. The search path is for MATLAB code, not for accessing datafiles.
I strongly recommend not adding folders of datafiles to the MATLAB path, it will slow down MATLAB.
The simple and efficient approach to access datafiles is to use absolute/relative filenames. MATLAB functions that import/export/read/write datafiles accept relative/absolute filenames. You will find FULLFILE very useful for that.
"Is there any way to read these files without having to add their directory to Matlab's search folder?"
Absolute/relative filenames. MATLAB should be able to access any file that is available to the user running MATLAB.
Note that you probably want UIGETFILE to return its second output, otherwise you will have a lot of difficulties.
