To create file associations from the command prompt, you may need one or both of, eg
CODE
assoc .rex=Regina
ftype Regina=e:\programs\exe\regina.exe %%1 %*
To make the association more permanent (that is, every time you boot BartPE), you need to modify the source .inf file.
CODE
[Software.AddReg]
; --- REX ---
0x1,"Classes\.rex","","rexxfile"
0x1,"Classes\rexxfile","","Regina Rexx"
0x2,"Classes\rexxfile\DefaultIcon","",""%SystemDrive%\Programs\exe\regina.exe",1"
0x2,"Classes\rexxfile\shell\open\command","","""%SystemDrive%\Programs\exe\regina.EXE"" ""%1"" %*"