I have adapted this answer from an out of date blog post where the links don't work, so I will provide all the instructions you need here.
Source: http://www.addictivetips.com/windows-tips/add-remove-any-folder-to-path-variable-using-context-menu/
WARNING: This procedure involves registry editing. If you are not comfortable editing the registry, turn back now. As you probably know, you can cause serious problems in the registry.
This procedure is written to work on Windows 7 or 8. I can't say how it will work outside of those versions.
Download the GTools utility pack from here: http://p-nand-q.com/download/gtools/
It's really just the "pathed.exe" tool you're looking for. For the purposes of this effort, you can remove everything else that's included in GToools.
Install the GTools or just place the pathed.exe file somewhere convenient and permanent.
Determine the path to the pathed.exe file. You will need it shortly.
Add a key to the windows registry at the following path:
"HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shell\"
Call it something that makes sense like "Add To Path"
Add another key under that key, called "command".
Within that key, set the default value similar to the following:
"C:\Program Files (x86)\GTools\pathed.exe" /APPEND "%0" /USER
Remember that it must be set with exactly this format. The quotes are very important.
Where "C:\Program Files (x86)\GTools\" represents wherever you ended up putting the pathed.exe file.
Add another key to the windows registry at the following path:
"HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shell\"
Call it something that makes sense like "Remove From Path"
Add another key under that key, called "command".
Within that key, set the default value similar to the following:
"C:\Program Files (x86)\GTools\pathed.exe" /REMOVE "%0" /USER
Remember that it must be set with exactly this format. The quotes are very important.
Where "C:\Program Files (x86)\GTools\" represents wherever you ended up putting the pathed.exe file.
Restart windows or go into task manager and restart "Windows Explorer" (a.k.a. explorer.exe)
This step is required to get Explorer to recognize the new context menu entries.
This places the chosen directory into the "User" path. Meaning it is specific to you. So, this will not affect other users.
It is important to note, that using this setup, it is easy to clutter up your user path, by placing directories in it multiple times. This doesn't have any real negative effect. It's just messy. This problem is easily resolved by using "Remove From Path" on any directory prior to using "Add To Path" on it. This would remove any prior entries of that directory and you would only end up with the one.
I think based on this, you can also figure out how to use pathed to place itself in the path, thereby allowing you to easily add directories to the path from the command line as well.
Should you ever wish to remove these context menu options, simply reverse the process.