I'm looking for a language that will let me do what I could do with Clipper years ago, and which I can do with Tcl, namely add functionality in a way other than just adding functions.

For example in Clipper/(x)Harbour there are commands #command, #translate, #xcommand and #xtranslate that allow things like this:

#xcommand REPEAT; 
   => DO WHILE .T. 
#xcommand UNTIL <cond>; 
  =>     IF (<cond>); 
             ;EXIT; 
         ;ENDIF; 
     ;ENDDO 

LOCAL n := 1
REPEAT
    n := n + 1
UNTIL n > 100

Similarly, in Tcl I'm doing

proc process_range {_for_ project _from_ dat1 _to_ dat2 _by_ slice} {
    set fromDate [clock scan $dat1]
    set toDate   [clock scan $dat2]

    if {$slice eq "day"} then {set incrementor [expr 24 * 60]}
    if {$slice eq "hour"} then {set incrementor 60}

    set method DateRange

    puts "Scanning from [clock format $fromDate -format "%c"] to [clock format $toDate -format "%c"] by $slice"
    for {set dateCursor $fromDate} {$dateCursor <= $toDate} {set dateCursor [clock add $dateCursor $incrementor minutes]} {
        # ...
    }
}
process_range for "client" from "2013-10-18 00:00" to "2013-10-20 23:59" by day

Are there any other languages that permit this kind of, almost COBOL-esque, syntax modification?

If you're wondering why I'm asking, it's for setting up stuff so that others with a not-as-geeky-as-I-am skillset can declare processing tasks.