You can import from DWG or DXF into schematics and use that to create symbols for the library. However, attempting to use DXF from our previous program was a fiasco. Every shape was a group of individual line entities and there were many entities that were completely invisible but killed all editing of properties of others if they were accidently included in the selection. DXF outputs are really dependent on how well the software was written, you may have better luck.
DWG should be more consistent since it is a more restricted drawing type.
Once you have a standard set of geometry saved as a general design sheet, you can edit it for the specific component and copy it to the symbol design. It saves a lot of work compared to editing in the symbol design sheet.
Take this chance to do any clean up of your symbols and use features of them that are available in Schematics. Ours are tied to our part numbers for the components and include the information to pull in the correct connector for the component and the correct terminals to fit the connector and wires.
We did this to save daily time for our harness designers at the expense of one time work for the librarians.
