When accessing a model in a DGNLIB that is not a print model, the Print Preparation task list will appear as shown with the only option being the tool to create a new sheet model.

If the current model is a valid Print Preparation sheet model then all of the Administration tools will be shown.

Select the Create new sheet model tool to create a new sheet model in the current DGNLIB. Toggle off Can be placed as a cell. Use any of the other options normally used for a sheet model. Make sure the Type is a Sheet model, as opposed to a Design model (from a seed file or not) and usually 2D. It is possible to set the size options now or use the tools later to reset.