run this command path "%path%;C:\foo"
documented help for path PATH /? Displays or sets a search path for executable files. PATH [[drive:]path[;...][;%PATH%] PATH ;
Type PATH ; to clear all search-path settings and direct cmd.exe to search only in the current directory. Type PATH without parameters to display the current path.
Including %PATH% in the new path setting causes the old path to be appended to the new setting.