[Logician]

If you mean, users pay you (via paypal, 2checkout.com etc.) then email you that they paid, you confirm the payment and manually give them rights (say by changing usergroup), you dont need a hack. Just add the link in your forum for donations and you are all set.

But if you are after a professional system where all these steps handled by a script, it is not easy for you to find such a hack. That is because:
a) The scripts entire code will change according to the service you are using. Say if you want to use paypal backbone for payments, script should be written according to this but if you signed up to say 2checkout.com, code should be rewritten. There are many transaction servers around where admins can sign up, so you can not have a "magical" hack to cover everyone's needs.
b) Many admins have different plans for paid & free members. Some want no access to forum until user has paid, some want partial access. Some admins want to restict feature X for free users, others want to grant feature Y for paid ones. Most of these special requests require more hacking on the site so you can not have one hack that satisfies everybody. Every admin has different needs on this integration.

If you just need to be donated by a few members, just follow my former suggestion and try to be happy with manual administration until your forum makes funds to get a professional project coded according to your needs.