I have an excisting clasifieds with a SQL database. I am buying some new software that uses the same database software. Is it easy to have the name and logins integrated from the existing database? Has anyone had experience doing this?

How easy it is depends on the software and how familiar you are with it. Yes it can be done but it is almost certainly not going to be a very simple job.

It can be done, I have done something similar myself. It may not be incredibly easy, but don't let the last poster put you off, if you have a bit of knowledge you should be fine.