I use Bonita BPM (opensource) for some business processes, in fact they already have one onboarding example (http://www.bonitasoft.com/for-you-to-read/process-library/employee-onboarding). But mine is heavily tweaked. That's what we do:
1-We connect Bonita to HR database, so we track down any new employee request (not onboarding, requests). The requester must declare what resources the new employee will need.
2-Once we have a employee request approved by CEO, we started buying stuff. We still don't know the end user, but we start provisioning computer, licenses, (plus cell phones, desk space, etc..)
3-When the new employee is defined, we take a lot of data from HR DB, and just ask for some info we don't have.
4-Once HR fill all what we need we automatically create all accounts (AD, mail) based on the username (checking for duplicates) and send and onboarding welcome mail. This email points to our intranet, where we have all the info about QA and IT. I think it's important to send a welcome mail with all the basic stuff, and the email must be automatic.
Before that we don't have any input of newcomers, they just appear in the office and we don't have any provision (we aren't allowed to have stock).
A BPM is very powerful, but it can be hard and tedious to implement on a company. But if you are sucessful implementing it, you'll save a lot of time.
We had peaks of about 30 newcomers in just 2 weeks (for 4 IT staff doing many other things), so we need to ease up this process. If your bussiness doesn't have a high rotation maybe all this is overkill.