You can use the generic Number fields to store and track billable hours but
I suggest you exercise caution in how far you go with that. Project is
first and foremost a work scheduling and cost estimating tool, NOT an
accounting application. Here's the problem - I have a task that's 5 days in
duration with one resource working full-time on it. The duration is 5 days
(40 hours) and the work is also 40 man-hours. If the resource earns $10 per
hour, the cost is $400. But how does that relate to billable hours?
Depending on your contract arrangements, etc, many different situations may
occur, varying even from task to task within the same project. Let's say it
was a fixed price contract - your billable amount might be $500. Your
resource finishes in half the time. Your cost is $200 but your billable is
$400. Or the resource takes half-again longer than expected - now your cost
may be $600 but the billable is still ... $500! OR consider the situation
of where you bill in whole day increments - resources works for 2 hours,
billable hours = 8 ... resource works for 8, billable hours = 8...or your
resources works 12, billable hours = 8 ... yet you only pay him for the
actual time he puts in. And there are many, many other scenarios that are
possible. My advice - use a real accounting package with a time and billing
module to track hours, payroll, client billings, etc and use Project for
what it does best - creating the task schedule and estimating and tracking
your internal direct costs of performing the work and purchasing the
materials required to complete your projects.