How can I import an iDVD project into DVD Studio Pro?
This seems to be a way to go about it, but this only seems to work for iDVD 4.0 or earlier.

I'm not sure how relevant this is to you though, because I don't know the versions of the software you are using.

I have zero experience working with iDVD but maybe it is possible to save your project in an older format / convert it to an older format, to make sure it can be imported.