Can I define my own building constructions for use in a single model?

Yes. To define a new wall construction (for example) for a model, open the model, click on the components tab in the Navigator panel on the left of the screen, select the category of wall you would like to add and click on the 'Add' toolbar button immediately above the Navigator window and define the construction data in the dialog. Once you have created the construction you will be able to reference it in the Constructions tab of model data.

Note that you will not be able to reference this model construction data from other models unless you first export it and import it into the other model.