How about an [+] Button to insert TC-variables into the currently active field?
Or at least show a little window to copy and select a variable.
Also, on "[Add] a new button", I would like to suggest to pre-fill out
the "Icon file:" field with "%Commander_Path%\WCMICONS.DLL"
(I hate to do this always by hand

Or perhaps add an [!] button to insert that string.
[+]
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%COMMANDER_PATH%
%COMMANDER_EXE%
%COMMANDER_DRIVE%
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%$APPDATA%
%$COMMON_APPDATA%
%$COMMON_DESKTOPDIRECTORY%
%$COMMON_DOCUMENTS%
%$COMMON_PICTURES%
%$COMMON_PROGRAMS%
%$COMMON_STARTMENU%
%$COMMON_STARTUP%
%$DESKTOP%
%$FONTS%
%$LOCAL_APPDATA%
%$MYMUSIC% (TC 7, XP only)
%$MYPICTURES%
%$MYVIDEO% (TC 7, XP only)
%$PERSONAL%
%$PROGRAMS%
%$STARTMENU%
%$STARTUP%
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