As long as it remains a single brigade, it will not be treated as a division. There are two ways to add another brigade, which will automatically convert it into a division. The first way is to deploy a freshly built brigade to the existing brigade from the deployment destination list. The second way is to put a second brigade or another division in the same province and "Reorganize" the two. You may need to use the "r" key, particularly if one of the "brigades" is a HQ, since the "Reorganize" button does not appear in some cases. Once it's got two brigades, it becomes a division and you can assign a General.