Hello bykfpuzz,
Regrettably there is not such feature to add the shortcut of Join Manager at the present time.
But as the temporary solution you can open it directly using command line mode.
1) Open Join manager and create join project file (in my case it will be List9.jxtl).
2) Delete lines with information about input and output files. It should be as below. Save it.
<timelines version="2" >
<timeline>
<group output="" >
<task type="joining" />
<track video="-1" audio="-1" >
</track>
</group>
</timeline>
</timelines>
3) Open Far Manager program, browse to the folder with installed Video Splitter and specify the path to the Video Splitter exe file and path project file of join manager in "". In my case it will be as on the picture below:
4) Press "Enter" button. Video Splitter with Join Manager will be opened.