Yes you can, slash commands work like this:
The function itself is in SlashCmdList["UniqueString"]
The "/..." is in SLASH_UniqueString1 = "/..." so all you got to do is find what "UniqueString" is and add one number higher and your own "/..." for example:
SLASH_UniqueString2 = "/mycmd"
In GlobalStrings.lua you got:
SLASH_USE2 = "/use";
SLASH_CAST4 = "/spell"; -- aka /cast
SLASH_CASTRANDOM2 = "/castrandom";
SLASH_CASTSEQUENCE2 = "/castsequence";
SLASH_CANCELAURA2 = "/cancelaura";
SLASH_CANCELFORM2 = "/cancelform";
(and many more)
For to have an alias for /use you could:
SLASH_USE3 = "/u"
I believe it would work