There are several WinAPI functions assigned to work with environment variables:

DECLARE INTEGER GetEnvironmentVariable IN kernel32;
	STRING lpName, STRING @lpBuffer, INTEGER nSize

DECLARE INTEGER SetEnvironmentVariable IN kernel32;
	STRING lpName, STRING lpValue

DECLARE INTEGER GetEnvironmentStrings IN kernel32

DECLARE INTEGER FreeEnvironmentStrings IN kernel32 INTEGER lpszEnvBlock

DECLARE INTEGER ExpandEnvironmentStrings IN kernel32;
	STRING lpSrc, STRING @lpDst, INTEGER nSize

The list is not complete.

* * *
Reading and setting Environment variables
http://www.news2news.com/vfp/?example=152

Sending environment strings into a cursor
http://www.news2news.com/vfp/?example=89

One more way to retrieve environment strings
http://www.news2news.com/vfp/?example=132