Without the use of AD the only way is to write an application to synchronise them for you.
I would set up a VM with active directory on, to handle this, so you have it there, just for this period, I really hope this is possible for you.
The good news is the code for doing this as an application would be almost identical on 2007/2010 so a windows form application would be possibly the best way to go if AD is NOT possible for you at all.
I would estimate 1 to 2 weeks for testing and development of such an application.
Hi Micheal, sure.
What you want is to create a tool that backs up all the users from the UserProfileManager.
This will then create your flat file, not know how it is setup you can design this flat file or temporary storage, or even just store as a stream.
This in turn can then be used to create ( http://msdn.microsoft.com/en-us/library/microsoft.office.server.userprofiles.userprofilemanager.aspx ) Replace Update or Delete CRUD the users.
I would put the time into testing it, even though getting this working may only take a day, planning I would allocate a day, full development 4 days but testing and tweaking a further 4 days.
I have done this for purely 2010 with AD but 2007 -> 2010 or even 2007 <-> 2010 wouldn't be much different.
To do the permissions, you simply need to check each site collections permission per user and create yourself a grid, adding the same permissions with your CRUD system of design to the opposing site collection, http://stackoverflow.com/questions/3940917/checking-permissions-of-a-user-with-a-site-collection