Use setx thus:
setx %PATH% "%PATH%;C:\somefolder"
setx is built into
cmd.exe. It can be used to make changes to a remote computer too.
setx modifies the “persistent” store though, not the “active” store (to borrow a terminology from
netsh. What this means is that the changes by
setx only take place when you open a new command prompt, it doesn’t affect the existing one.