I don't think they will try to help you with this

. Did you at least keep a copy of your old smf and joomla DB? If yes you can use that to restore your users. The problem with that service you used and I used it as well in the case for trying out fireboard is that I didn't want it to touch my users or their information. Using mtwmigrator I used that to port my users from joomla 1.0.15 to joomla 1.5.3 and it worked great. Sync fireboard and you should be good to go.
My last bit of advice is this. If you are running Joomla 1.5.3 or any 1.5 series stay away from fireboard for awhile till they get things in gear. Joomla 1x fine use it but if you go 1.5 you were warned. The developers here seem reluctant to add any type of support for the 1.5 series from my reading and the legacy support is ineffective. Joomla 1.6 is going to do away with legacy and guess what? If you aren't with the boat when it leaves then I guess you are stranded huh?
Personally the only reason to use fireboard is to not modify any files with your joomla install. There are just too many tried and true forums out there and once the devs here actually get everything to work it will make a good vanilla forum choice for any joomla site. If and a big if at that the joomla team with phpbb decide to release some type of integration support well that's it. It will probably happen since joomla would be a very very attractive choice for almost any type of site at that point.