In OpticStudio, We can load any spectrum file from our   \Objects\Sources\Spectrum Files Folder .Here is the available code for Matlab and Python. You can use this code in Standalone or Interactive Extension.

-Copy the code in your file .

-Change the path of testfile

Matlab Code: 

import ZOSAPI.*;

TheSystem = TheApplication.PrimarySystem;

sampleDir = TheApplication.SamplesDir;

testFile = System.String.Concat("C:\test\ZPO\Diode sample.ZMX");   

NSCE = TheSystem.NCE;

Obj1 = NSCE.GetObjectAt(1);  

Source_Color=Obj1.SourcesData.SourceColor;

Source_Color=ZOSAPI.Editors.NCE.SourceColorMode.SpectrumFile;

filesAvailable = Obj1.SourcesData.SourceColorSettings.GetAvailableSpectrumFiles();

FileToSelect=filesAvailable(6);

Obj1.SourcesData.SourceColorSettings.SpectrumFile=FileToSelect; 

Python Code:

testFile = r"C:\test\ZPO\Diode sample.ZMX"

TheSystem.LoadFile(testFile, False)

print(TheSystem.SystemFile)

NSCE = TheSystem.NCE

Obj1 = NSCE.GetObjectAt(1) 

Source_Color=Obj1.SourcesData.SourceColor

Source_Color=ZOSAPI.Editors.NCE.SourceColorMode.SpectrumFile

filesAvailable = Obj1.SourcesData.SourceColorSettings.GetAvailableSpectrumFiles()

Obj1.SourcesData.SourceColorSettings.SpectrumFile=filesAvailable[1]

print(Obj1.SourcesData.SourceColorSettings.SpectrumFile)