I found an additional way how to use the merged files: But for that you need to be familiar with script files. It is also a good way to automate the entire steps.
Use for that the 'Surface selection'-tool. Select then the torso of the figure you have created the TF_*.*-Files and save the selected item with Files/Save as ... as Shader preset. Take care, that you do not save it as 'compressed'.
Open this file now with an editor like NotePad++ (it should be able to handle UNIX like files, the default windows NotePad is not the best option)
The most easiest way is, if you save your merged files in the same folder where the files for the torso are located, but replace the word 'Torso' with 'Genitalia':
If - for example - your TF-File is named tf_3duG3MTG2_Torso_DF_1002.jpg you should save the merged file as 3duG3MTG2_Genitalia_DF_1002.jpg. Move those files to the folder where the torso files are located. You will find the location in the Duf-File for the shader preset.
If you did this, replace in the Duf-File the word 'Torso' with 'Genitalia' and save it.
Then you can move this file to the location of the texture for your figure without genitalia (Select the figure in the content library, go to the material-preset, do a right click on a material file and select 'Browse to File location ...', if you want, you can create also a sub directory there).
Do a right click afterwards in the area for the material files, but you must not be on one icon, then you get the option 'Refresh'. Click on it and you will see the Duf-File.
Select then - still with the 'Surface selection'-tool - the genitalia and click on the Duf-File and the textures are loaded. Then you can render it and save the output as *.Png-File. Move this file also to the location where the Duf-File is. Give both the same name with different extensions. Refresh your material zone and it looks like the genitalia are original available.
This works only, if the 'Torso-Files' contain 'Torso' and you want to have the texture files in the same folder like the torso-files.
Especially for the male genitalia we have the problem, that we may have an uncut and circumcised version. In this case you either have to place the texture files in different folders or give them different names. See below how to deal with this:
If you want to place 'Your' files in a separate sub folder, you have to do some steps more with the Duf-file:
In the part "image_library" you will see some "url"'s which represent the location of the torso files. Use the replace function of the editor and replace all occurrences of a string like '/Toon%20Generations/Figure/3duG3MTG2_Torso' with a string like '/Toon%20Generations/Figure/Gen/3duG3MTG2_Torso'. Do not replace 'Torso' here! Depending on the shader setting this path might be used also outside of the part "image_library", so do not try to search them manually.
Then you have to replace all (up to 5) names of the torso textures with the name of your new genitalia textures, means: Diffuse to Diffuse, Bump to Bump. If you named your files consequent (you named the merged files like the tf_-File without the leading 'tf_' and replaced the alias for torso with always the same alias for the genitalia, e.g. 'tf_3duG3MTG2_Torso_DF_1002.jpg' to '3duG3MTG2_GenCir_DF_1002.jpg' and 'tf_3duG3MTG2_Torso_SP_1002.jpg' to '3duG3MTG2_GenCir_SP_1002.jpg' it is sufficient to replace only '_Torso_' with '_GenCir_'.
Finally you have to search for a block "material_library" and replace below 'Torso' with 'Genitalia' and also in the part "scene" / "materials" '#Torso' with '#Genitalia'.
That's it!