You can declare the empty string as separator:
separator Ident ""
In practice this lets you use white-spaces (or any space character) as separator:
$ cat test.cf
A . A ::= [Ident] ;
separator Ident ""
$ bnfc -haskell -m test.cf
$ make
$ echo 'x y z' | ./Testtest
Parse Successful!
[Abstract Syntax]
A [Ident "x",Ident "y",Ident "z"]
[Linearized tree]
x y z