Maybe something like:

Code:
On *:Text:!loan & &:#Loans: {
  var %msg = $nick wants $2 at $time on $date for $3
  if ($read(loans.txt,w,$+(*,$nick,*))) {
    write $+(-l,$readn) loans.txt $nick wants $calc($gettok($ifmatch,3,32) + $2) at $time for $3
    msg $chan Updated loan: $nick: $read(loans.txt,$readn)
  }
  else {
    write loans.txt %msg
    msg $chan %msg
  }
}


[16:27] <SladeKraven> !loan 50 1Week
[16:27] <Andy> SladeKraven wants 50 at 16:27:25 on 15/08/2005 for 1Week
[16:27] <SladeKraven> !loan 50 1Week
[16:27] <Andy> Updated loan for SladeKraven wants 100 at 16:27:32 for 1Week

As you can see it adds to the old loan.

-Andy