The converter for phpBB 1.8 is here : viewtopic.php?f=486&t=2278541
, it's for phpBB 3.1, there isn't any converter for MyBB 1.8 > phpBB 3.0
However, it is no working...
I managed to have a conversion from a mybb 1.8 with the previously script you used, don't remember which solution worked: delete the part about "reportedpost" table importation (as i was: do not need this) in the script or copie the table under this name (it changed for mybb 1.8, google about it you should find it).
Anyway, I did not check if the whole conversion was good in the end and there was still the problem for images attachment reported in the end of the thread.
It would be better to have a word from prototech but he seems pretty far away~~ There is not too much support for the converters :/