Currently, equipment items can only be linked to the BIM model if the equipment item was generated from the equipment model. On a large percentage of projects, the final BIM model does not precede the need for having an equipment item within Field. You should be able to assign a BIM object to a piece of equipment after the fact.
Also, BIM objects (and their Identifiers) can change frequently as projects progress. You should be able to unlink an existing BIM-equipment link, and relink it to a different object.