Not sure how to accomplish this with VBA.
However, it's do-able with Macro Express Pro. For example, you could build a hotkey-activated macro that navigates to the "Subject:" field, places the cursor at the start of the field, types "00 ", and backspaces once. At that point, all you need to do is type "87" or whatever the next number is.
This may not be the easiest project for a novice, but I know it can be done: I've got a similar macro that navigates to the "From:" field to extract the email address.
An evaluation copy of Macro Express Pro is available from
Windows Automation - Macro Express. It will work for 30 days. If you want to continue using the program afterwards, the cost is a modest $80.
The non-macro alternative is to press
Shift + Tab a bunch of times.