If you absolutely NEED ARexx, it wouldn't take much to convert this, but here goes...
Edit - Formatting got a bit fried. Oh well, if it's on track and you can't make heads or tails, you know where to find me. ;-)
.KEY SOURCE/A,LHAFILE/A,SINCEDATE/K,ALL/S,SETARCBIT/S
.BRA {
.KET }
.DEF ALL ""
IF "{SinceDate}" NOT EQ ""
LIST TO T:ArcFileList{$$} "{Source}" {All} FILES SINCE {SinceDate} LFORMAT "%p%n"
ELSE
LIST TO T:ArcFileList{$$} "{Source}" {All} LFORMAT "%p%n"
ENDIF
; Assuming LhA is in your path
LhA -iRAM:ArcFileList{$$} a "{LhAFile}"
IF "{SetArcBit}" NOT EQ ""
LIST TO T:ArcBitList{$$} "{Source}" {All} FILES SINCE {SinceDate} LFORMAT "protect %p%n +a"
EXECUTE T:ArcBitList{$$}
ENDIF
DELETE T:ArcFileList{$$} T:ArcBitList{$$} QUIET
