It is possible.
To add commands, open the file LaTeX.dic (you can find it in %B\Dict\LaTeX folder), add all the commands you want (one per line).
Then sort the file as a dictionary (Tools -> Sort Lines).

Choose "Dictionary" as a Compare Table, press Sort and then OK.
Save the file and restart WinEdt.
To add more environments, it is a bit more complicated. Do the following:
Show the "Options Interface" (Options -> Options Interface)

Double-click the "Auto Complete" item (AutoComplete.ini gets opened)
Just before the line
END="LaTeX Environments"
add the following lines
OBJECT="definition"
MACRO="LetReg(-1,!' ');"+ // Content placeholder (optionally indented)
"Exe('%b\Macros\Auto Complete\TeXEnv.edt');"
Substitute definition and do the same for other environments you want to add.
Press Shift+Ctrl+F9 to reload the file.
That's all.
propositionandlemma. – karlkoeller Jan 04 '18 at 12:17