I don't think that creating a "item number" attributes is a good idea. As out of the box, the number of the WTPart is the standard unique primary key . And as you say, the most exposed attribute in Windchill
And the WTPart is clearly the PLM side twin of the ERP Item. So it should be the same number in both system
You can configure your Windchill in OIR's to allow manual or automatic numbering (for example based on a subtype or folder ...).
And for better integration, you can customize your own number generator (for example a webservice, depending of your ERP capabilities) to pick the number directly in the ERP sequence.
Or resolve it just by process. If your need is to build WTpart BOM from CAD, then publish it to ERP . The PLM becomes the ERP number generator ...