You could usethe auto enter expression: Self * 1000
or Self & "000"
To keep repeated edits from adding 3 zeroes each time, you can include an If function:
If ( Self < 1000 ; Self * 1000 ; self )
In all of the above, number formatting on the field could then add the thousands separator.
If it's just "for looks", you could even place your number on the layout as merge text followed by ",000"
This last option can't be edited, but it works for reports and such.