The Authen::LTIBasic module is no longer being actively maintained, as it has been superseded by the Authen::LTIAdvanced module.
I took a quick look at the code, and it looks like the LTIBasic module is hard-coded to not create user accounts for permission levels higher than TA, so the only way to change this would be to edit the code.
The default behaviour is the same for the LTIAdvanced module, but in that case there is a configuration option which allows you to change the maximum permission level for which accounts are automatically created.
It's also worth noting that this behaviour was deliberate, as it prevents users who have obtained professor permissions in the LMS from automatically getting professor permissions in WeBWorK.